mtd: nand: do not check R/B# for CMD_READID in nand_command(_lp)

Read ID (0x90) command does not toggle the R/B# pin.  Without this
patch, NAND_CMD_READID falls into the default: label, then R/B# is
checked by chip->dev_ready().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>