MIPS: Alchemy: Fix unchecked kstrtoul return value

enabled __must_check logic triggers a build error for mtx1 and gpr
in the prom init code.  Fix by checking the kstrtoul() return value.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/6574/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>