staging: mt7621-mmc: Correct spelling mistakes in comments

Changed "avaiable" to "available" and "interupt" to "interrupt".

Signed-off-by: Jona Crasselt <jona.crasselt@fau.de>
Signed-off-by: Felix Windsheimer <felix.windsheimer@fau.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>