ARM: mx28: check for gated clocks when setting saif divider

Like with all other clocks, the divider for the SAIF devices should not
be altered when the clock is gated. Bail out when this is the case like
the other clocks do.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Dong Aisheng-B29396 <B29396@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>