mtd: nand: fsmc: remove ->select_bank() from fsmc_nand_platform_data

Since commit 4404d7d8 ("mtd: nand: fsmc: remove stale non-DT probe
path"), only DT probing is used for the fsmc_nand driver. Due to this,
the ->select_bank() field of fsmc_nand_platform_data is never used, so
this commit gets rid of it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>