staging: mt7621-mmc: Remove unused variable from msdc_tune_request

The variable cmd in the function msdc_tune_request is set but never
used, so remove it.

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