staging: mt7621-mmc: Relax cpu while waiting for stable clock

Current code just busy waits with nop for clock becoming stable,
relaxing the cpu here improves readability and portability and also
removes a unnecessary difference with the mtk-sd driver.

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