drm/amd/display: fix signedness bug in execute_synaptics_rc_command()

The "ret" variable needs to be signed for the error handling to work.

Fixes: 2ca97adc ("drm/amd/display: Add Synaptics Fifo Reset Workaround")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>