staging: mt7621-mmc: Inline function msdc_set_blknum

The function msdc_set_blknum consists of one (real) line of code and
is only called once, so inline it makes the code shorter and more
readable.

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