crypto: marvell/cesa - factor out first fragment decisions to helper

Multiple locations in the driver test the operation context fragment
type, checking whether it is a first fragment or not.  Introduce a
mv_cesa_mac_op_is_first_frag() helper, which returns true if the
fragment operation is for a first fragment.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>