staging: mt7621-mmc: Factor out from msdc_command_start()

Currently msdc_command_start does multiple things and is hard to read,
so factor out the finding of the response type.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>