bcma_mips: add a const qualifier

Be a bit stricter and add few more 'const' qualifiers.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: "Rafał Miłecki" <zajec5@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>