[media] s5p-mfc: Fix an error check

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.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>