mt76: mt7915: get rid of mt7915_mcu_set_fixed_rate routine

Fix the following sparse warning removing mt7915_mcu_set_fixed_rate
since it is no longer used:
warning: symbol 'mt7915_mcu_set_fixed_rate' was not declared.
Should it be static?

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>