MIPS: Fix CONFIG_MIPS_CMDLINE_DTB_EXTEND handling

The CONFIG_MIPS_CMDLINE_DTB_EXTEND option is used so that the kernel
arguments provided in the 'bootargs' property in devicetree are extended
with the kernel arguments provided by the bootloader.

The code was broken, as it didn't actually take any of the kernel
arguments provided in devicetree when that option was set.

Fixes: 7784cac6 ("MIPS: cmdline: Clean up boot_command_line initialization")
Cc: stable@vger.kernel.org
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>