ARM: mxs: correct the using of frac div for saif

According to spec, set to 1 is the enable of fractional devide
or the clock can not be generated properly.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>