mt76: move mt76x02_mac_set_short_preamble in mt76x02_mac.c

Move mt76x02_mac_set_short_preamble routine in mt76x02-lib module
since it is shared between mt76x0 and mt76x2 drivers

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>