Minor changes to sha256.c to bring it in line with sha512.c
Minor changes to sha256.c to bring it in line with sha512.c

Signed-off-by: Tom Cosgrove <c6e7a4d0c027d32c62bc9570c565a01587ef99be@arm.com>

Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls