mx5/clock: fix clear bit fields issue in _clk_ccgr_disable function

We can see MXC_CCM_CCGRx_MOD_OFF is defined as 0 while
MXC_CCM_CCGRx_CG_MASK is defined as 0x3 in crm_regs.h, here in the
_clk_ccgr_disable function, we want to clear the corresponding enable
bit fields to disable this clock, so we should choose MASK instead of
OFF otherwise clocks can't be disabled.

Signed-off-by: Jason Wang <jason77.wang@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>