wifi: mt76: mt7915: Fix chainmask calculation on mt7915 DBDC

mt7915 does not have a per-band number of chains unlike the other chips,
it only has a total number of chains.  Yet the current code would
consider the total number as a per-band number.

For example, it would report that a 2x2 + 2x2 DBDC card have 4 chains on
each band and set chainmask to 0b1111 for the first interface and
0b11110000 for the second.

Fixes: 99ad32a4 ("mt76: mt7915: add support for MT7986")
Co-developed-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Felix Fietkau <nbd@nbd.name>