staging: mt7621-mmc: Make msdc_clr_fifo a function and relax cpu

In the current code, msdc_clr_fifo is a macro and just busy waits for
a limited amount of time for the fifo clear to finish. That is not
correct, the programming manual hits, that the user should wait until
the bit is cleared by hardware and not a limited amount of time.

So the code is changed to a function, that also relaxes the cpu while
busy waiting.

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