s390/crypto: Fix unsigned variable compared with zero

s390_crypto_shash_parmsize() return type is int, it
should not be stored in a unsigned variable, which
compared with zero.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 3c2eb6b7 ("s390/crypto: Support for SHA3 via CPACF (MSA6)")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Joerg Schmidbauer <jschmidb@linux.vnet.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>