mtd: mxc_nand: add missing clk_disable_unprepare() in mxcnd_remove()

clock source is prepared and enabled by clk_prepare_enable() in
mxcnd_probe() function, but no disable/unprepare in mxcnd_remove().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>