wifi: mt76: mt7915: fix band_idx usage

The commit 006b9d4a introduced phy->band_idx to accommodate the
band definition change for mt7986 so that the band_idx of main_phy
can be 0 or 1. Accordingly, the source of band_idx 1 has switched to
"phy != &dev->phy" or "dev->phy.band_idx = 1".

We still use ext_phy to represent band 1 somewhere in driver, so fix it.
Also, band_idx sounds more reasonable than dbdc_idx, so change it.

Fixes: 006b9d4a

 ("mt76: mt7915: introduce band_idx in mt7915_phy")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>