mips: kgdb: adjust the comment to the actual ifdef condition

The comment refers to CONFIG_CPU_32BIT, but the ifdef uses CONFIG_32BIT.
As this ifdef and comment was introduced with initial mips-kgdb commit
88547001 ("[MIPS] kgdb: add arch support for the kernel's kgdb core"),
it is probably just a minor issue that was overlooked during the patch
creation and refactoring before submission.

This inconsistency was identified with ./scripts/checkkconfigsymbols.py.
This script has been quite useful to identify a number of bugs with
Kconfig symbols and deserves to be executed and checked regularly.

So, adjust the comment to the actual ifdef condition 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>