MIPS: Fix macro typo

Change the CONFIG_MIPS_CMDLINE_EXTEND to CONFIG_MIPS_CMDLINE_DTB_EXTEND
to resolve the EXTEND_WITH_PROM macro.

Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Fixes: 2024972ef533 ("MIPS: Make the kernel arguments from dtb available")
Reviewed-by: Alexander Sverdlin <alexander.svedlin@gmail.com>
Cc: Jonas Gorski <jogo@openwrt.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Aaro Koskinen <aaro.koskinen@nokia.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11909/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>