mt76: mt7615: add missing declaration in mt7615.h

Add mt7615_mcu_wait_response declaration in mt7615.h since it will be
reused adding usb support to mt7615 driver

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 044a4325 ("mt76: mt7615: introduce mt7615_mcu_wait_response")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/d341335a636b6ccd088dd2cfeec2d296eb4dc8c7.1584534454.git.lorenzo@kernel.org