hash: Drop some #ifdefs in hash.c

We can use the __maybe_unused attribute to avoid some of the #ifdefs in
this file. Update the functions accordingly.

Note: The actual hashing interface is still a mess, with four separate
combinations and lots of #ifdefs. This should really use a driver
approach, e.g. as is done with partition drivers.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>