if_ffec: fix imx6sx compatible flags
The i.MX 6 SoloX does support Gigabit and AVB features as can be seen in
its technical reference manual [1] or even Linux kernel [2].
Fixes: 94bc2117b4a ("Add i.MX 8M Quad support")
[1] https://www.nxp.com/webapp/Download?colCode=IMX6SXRM
[2]
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/net/ethernet/freescale/fec_main.c?h=linux-6.6.y#n206
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/1479
Signed-off-by: Gary Bisson <gary.bisson@ezurio.com>