wifi: mt76: mt7915: split mcu chan_mib array up

The current flow confuses coverity check that leads to false reporting,
so split the offs[] into two pieces according to chipset revision to
silence coverity tool.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>