mips: dec: provide the correctly capitalized config CPU_R4X00 in init error message

mips: dec: provide the correctly capitalized config CPU_R4X00 in init error message

The config for MIPS R4000-series processors is named CPU_R4X00 with
upper-case X, not CPU_R4x00 as the error message suggests.

Hence, ./scripts/checkkconfigsymbols.py reports this invalid reference:

  CPU_R4x00
  Referencing files: arch/mips/dec/prom/init.c

When human users encounter this error message, they probably just deal
with this minor discrepancy; so, the spelling never was a big deal here.

Still, the script ./scripts/checkkconfigsymbols.py has been quite useful
to identify a number of bugs with Kconfig symbols and deserves to be
executed and checked regularly.

So, repair the error message to reduce the reports made the script and
simplify to use this script, as new issues are easier to spot when the
list of reports is shorter.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>