mtd: mxc_nand: fix unbalanced clk_disable() in error path

If nand_scan_ident() or nand_scan_tail() fails, the NAND chip may have
been deselected and the clock already disabled. Thus, check 'clk_act'
in the error path to decide whether the clock still needs to be
disabled.

This fixes a:
|WARNING: at drivers/clk/clk.c:472 __clk_disable+0x3c/0x78()

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>