Checking unsigned variable for negative value always returns false.
Hence make this value signed as we expect it to be negative too.
Fixes the following smatch warning:
drivers/media/platform/s5p-mfc/s5p_mfc_opr_v6.c:572
s5p_mfc_set_enc_ref_buffer_v6() warn: unsigned 'buf_size1' is never
less than zero.