Crypto api changes:
o don't use the key index to identify when the driver has been

asked to allocate a key slot, use an explicit flag; allows
drivers to force s/w fallback for entries in the global table

o change callback api to allocate driver resources for a crypto key:

• de-const the key parameter so drivers can muck with the flags
• on callback failure don't automatically try to setup s/w crypto; instead the driver must now mark the key entry for s/w crypto and the caller will re-attach the cipher module

NB: api change permits drivers more control over fallback to s/w

crypto (e.g. based on a limited number of h/w key slots)