mt76: mt7603: use the correct hweight8() function

__sw_hweight8() is only defined if CONFIG_GENERIC_HWEIGHT is enabled.
The function that works on all architectures is hweight8().

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>