Bluetooth: btmtk: Add MODULE_FIRMWARE() for MT7922

Since dracut refers to the module info for defining the required
firmware files and btmtk driver doesn't provide the firmware info for
MT7922, the generate initrd misses the firmware, resulting in the
broken Bluetooth.

This patch simply adds the MODULE_FIRMWARE() for the missing entry
for covering that.

Link: https://bugzilla.suse.com/show_bug.cgi?id=1214133


Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>