enic: make enic_dev_enable/disable ref-counted

enic: make enic_dev_enable/disable ref-counted

Both the data path (ndo_open/ndo_stop) and the upcoming admin channel
need to enable and disable the vNIC device independently. Without
reference counting, closing the admin channel while the netdev is up
would inadvertently disable the entire device.

Add an enable_count to struct enic, protected by the existing
devcmd_lock. enic_dev_enable() issues CMD_ENABLE_WAIT only on the
first caller (0 -> 1 transition), and enic_dev_disable() issues
CMD_DISABLE only when the last caller releases (1 -> 0 transition).

Also check the return value of enic_dev_enable() in enic_open() and
fail the open if the firmware enable command fails. Without this check,
a failed enable leaves enable_count at zero while the interface appears
up, which can cause a later admin channel enable/disable cycle to
incorrectly disable the hardware under the active data path.

Signed-off-by: Satish Kharat <satishkh@cisco.com>
Link: https://patch.msgid.link/20260401-enic-sriov-v2-prep-v4-4-d5834b2ef1b9@cisco.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>