eth: fbnic: Add support for HDS configuration

eth: fbnic: Add support for HDS configuration

Add support for configuring the header data split threshold.
For fbnic, the tcp data split support is enabled all the time.

Fbnic supports a maximum buffer size of 4KB. However, the reservation
for the headroom, tailroom, and padding reduce the max header size
accordingly.

  ethtool_hds -g eth0
  Ring parameters for eth0:
  Pre-set maximums:
  ...
  HDS thresh:		3584
  Current hardware settings:
  ...
  HDS thresh:		1536

Verify hds tests in ksft-net-drv are passing

  ksft-net-drv]# ./drivers/net/hds.py
  TAP version 13
  1..13
  ok 1 hds.get_hds
  ok 2 hds.get_hds_thresh
  ok 3 hds.set_hds_disable # SKIP disabling of HDS not supported by ...
  ...
  ...
  ok 12 hds.ioctl_set_xdp
  ok 13 hds.ioctl_enabled_set_xdp
  \# Totals: pass:12 fail:0 xfail:0 xpass:0 skip:1 error:0

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Link: https://patch.msgid.link/20250813221319.3367670-2-mohsin.bashr@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>