sb1250-duart: __maybe_unused, etc. fixes

This is a set of small fixes addressing points raised with the original
driver submission.  In particular, __maybe_unused is used rather than a
local hack and sbd_ops is made const.  Additionally I have made two local
string variables automatic as rodata space was wasted for pointers
unnecessarily.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>