staging: mt7621-mmc: Replace macro sdr_get_field with function

Currently sdr_get_field is a macro, to bring the code in line with the
upstream driver mtk-sd, it is changed to a function.

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