selftests: drv-net: stats: use skip instead of xfail for unsupported features

selftests: drv-net: stats: use skip instead of xfail for unsupported features

XFAIL is considered a form of a pass by our CI. For HW devices returning
XFAIL for unsupported features is counter-productive because our CI
knows not to expect any HW test to pass until it sees 10 passes in a row.
If we return xfail the test shows up as pass even if the device doesn't
support the feature. netdevsim supports all features necessary for
the stats test so there is no concern about running the test in SW mode.

Make the test skip rather than xfail if driver doesn't support FEC or pause.

Link: https://patch.msgid.link/20250620161109.2146242-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>