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

Set Features (0xEF) command toggles the R/B# pin after 4 sub feature
parameters are written.

Currently, nand_command(_lp) calls chip->dev_ready immediately after
the address cycle because NAND_CMD_SET_FEATURES falls into default:
label.  No wait is needed at this point.

If you see nand_onfi_set_features(), R/B# is already cared by the
chip->waitfunc call.

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