mt76: mt7915: get rid of dbdc debugfs knob

mt7915 automatically detects dbdc feature so drop debugfs knob.
Move if_comb, mt7915_rates, if_limits structs and mt7915_regd_notifier(),
mt7915_init_wiphy() routines at the begging of init.c in order to make
mt7915_register_ext_phy routine static

Co-developed-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Co-developed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>