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()