mt76: Use the correct hweight8() function

mt76_init_stream_cap() and mt76_get_txpower() call __sw_hweight8()
directly, but that's only defined if CONFIG_GENERIC_HWEIGHT is
enabled.  The function that works on all architectures is hweight8().

Fixes: 551e1ef4 ("mt76: add mt76_init_stream_cap routine")
Fixes: 9313faac ("mt76: move mt76x02_get_txpower to mt76 core")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Felix Fietkau <nbd@nbd.name>