dm-cache: remove pointless error check

Smatch reported following:
'''
drivers/md/dm-cache-target.c:3204 parse_cblock_range() warn: sscanf doesn't return error codes
drivers/md/dm-cache-target.c:3217 parse_cblock_range() warn: sscanf doesn't return error codes
'''

Sscanf doesn't return negative values at all.

Signed-off-by: Dipendra Khadka <kdipendra88@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>