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>

drivers/staging/mt7621-mmc/sd.c

@@ -618,7 +618,7 @@ static void msdc_set_mclk(struct msdc_host *host, int ddr, unsigned int hz)
 
 	/* wait clock stable */
 	while (!(sdr_read32(MSDC_CFG) & MSDC_CFG_CKSTB))
-		;
+		cpu_relax();
 
 	host->sclk = sclk;
 	host->mclk = hz;