MIPS: TXx9: CONFIG_TOSHIBA_RBTX4939 spelling

Fix a typo in the comment for the TOSHIBA_RBTX4939 config option

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>