mt76: usb: move mt76u_check_sg in usb.c

Move mt76u_check_sg routine in usb.c and introduce sg_en variable
in mt76_usb in order to check if scatter-gather is supported by
mt76u layer

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>