tpm: Use HMAC-SHA256 library instead of open-coded HMAC

Now that there are easy-to-use HMAC-SHA256 library functions, use these
in tpm2-sessions.c instead of open-coding the HMAC algorithm.

Note that the new implementation correctly handles keys longer than 64
bytes (SHA256_BLOCK_SIZE), whereas the old implementation handled such
keys incorrectly.  But it doesn't appear that such keys were being used.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>