net: ethernet: ti: netcp: Standardize knav_dma_open_channel to return NULL on error

net: ethernet: ti: netcp: Standardize knav_dma_open_channel to return NULL on error

Make knav_dma_open_channel consistently return NULL on error instead
of ERR_PTR. Currently the header include/linux/soc/ti/knav_dma.h
returns NULL when the driver is disabled, but the driver
implementation does not even return NULL or ERR_PTR on failure,
causing inconsistency in the users. This results in a crash in
netcp_free_navigator_resources as followed (trimmed):

Unhandled fault: alignment exception (0x221) at 0xfffffff2
[fffffff2] *pgd=80000800207003, *pmd=82ffda003, *pte=00000000
Internal error: : 221 [#1] SMP ARM
Modules linked in:
CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.17.0-rc7 #1 NONE
Hardware name: Keystone
PC is at knav_dma_close_channel+0x30/0x19c
LR is at netcp_free_navigator_resources+0x2c/0x28c

[... TRIM...]

Call trace:
 knav_dma_close_channel from netcp_free_navigator_resources+0x2c/0x28c
 netcp_free_navigator_resources from netcp_ndo_open+0x430/0x46c
 netcp_ndo_open from __dev_open+0x114/0x29c
 __dev_open from __dev_change_flags+0x190/0x208
 __dev_change_flags from netif_change_flags+0x1c/0x58
 netif_change_flags from dev_change_flags+0x38/0xa0
 dev_change_flags from ip_auto_config+0x2c4/0x11f0
 ip_auto_config from do_one_initcall+0x58/0x200
 do_one_initcall from kernel_init_freeable+0x1cc/0x238
 kernel_init_freeable from kernel_init+0x1c/0x12c
 kernel_init from ret_from_fork+0x14/0x38
[... TRIM...]

Standardize the error handling by making the function return NULL on
all error conditions. The API is used in just the netcp_core.c so the
impact is limited.

Note, this change, in effect reverts commit 5b6cb43b4d62 ("net:
ethernet: ti: netcp_core: return error while dma channel open issue"),
but provides a less error prone implementation.

Suggested-by: Simon Horman <horms@kernel.org>
Suggested-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20251103162811.3730055-1-nm@ti.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>