staging: mt7621-mmc: Remove multiple assignments

Fix checkpatch: multiple assignments should be avoided, to improve
readability.
It aslo moves the second assignment out of the if/else block, that is
valid, because drv_mode is of type msdc_mode, an enum with only those
three elements, so one of the if/ else if statements is always
taken. And the second assignment can happen after the conditions.

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