mtd: nand: pxa3xx: Use devm_clk_get

Replacing clk_get by managed devm_clk_get, the error path
can be greatly simplified.

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>