mtd: plat_nand: fix section error

With CONFIG_HOTPLUG=n, the following eror occurred during link:
local symbol 0: discarded in section `.devexit.text' from
drivers/built-in.o

It was caused by improper section reference. The __devexit_p()
should be added to the .remove function.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>