mtd: nand: gpmi: Kill gpmi_nand_exit()

The only user of gpmi_nand_exit() is gpmi_nand_remove(). Move its content
to the caller.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Han Xu <han.xu@nxp.com>