stmmac: intel: Fix warning message for return value in intel_tsn_lane_is_available()

stmmac: intel: Fix warning message for return value in intel_tsn_lane_is_available()

Fix the warning "warn: missing error code? 'ret'" in the
intel_tsn_lane_is_available() function.

The function now returns 0 to indicate that a TSN lane was found and
returns -EINVAL when it is not found.

Fixes: a42f6b3f1cc1 ("net: stmmac: configure SerDes according to the interface mode")
Signed-off-by: Choong Yong Liang <yong.liang.choong@linux.intel.com>
Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250310050835.808870-1-yong.liang.choong@linux.intel.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>