MIPS: Sort out CPU type to name translation.

As noticed by David Daney <ddaney@caviumnetworks.com>, the old long switch
statement did not comply with the Linux C coding style.  It was also yet
another place of code to be changed when adding a new processor type
leading to annoying bugs for example in /proc/cpuinfo.

Fixed by moving the setting of the CPU type string into the core of the
probing code and a few BUG_ON() test to ensure the CPU probing code indeed
did its job and removing multiple now redundant tests.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>