s390: crypto: Use min_t() instead of doing it manually

Fix following coccicheck warning:
drivers/s390/crypto/zcrypt_ep11misc.c:1112:25-26: WARNING opportunity for min()

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>