mtd: nand: pxa3xx: Check for clk_prepare_enable() return value

clk_prepare_enable() can fail due to unknown reason.
Add a check for this and return the error code if it fails.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>