crypto: x86/sha256-ni - rename some register aliases

MSGTMP[0-3] are used to hold the message schedule and are not temporary
registers per se.  MSGTMP4 is used as a temporary register for several
different purposes and isn't really related to MSGTMP[0-3].  Rename them
to MSG[0-3] and TMP accordingly.

Also add a comment that clarifies what MSG is.

Suggested-by: Stefan Kanthak <stefan.kanthak@nexgo.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>