drm/radeon: change function signature to pass full range

In function ‘radeon_process_i2c_ch’ a comparison of a u8 value against
255 is done. Since it is always false, change the signature of this
function to use an `int` instead, which match the type used in caller:
`radeon_atom_hw_i2c_xfer`.

Fix the following warning triggered with W=1:

  CC [M]  drivers/gpu/drm/radeon/atombios_i2c.o
  drivers/gpu/drm/radeon/atombios_i2c.c: In function ‘radeon_process_i2c_ch’:
  drivers/gpu/drm/radeon/atombios_i2c.c:71:11: warning: comparison is always false due to limited range of data type [-Wtype-limits]
   if (num > ATOM_MAX_HW_I2C_READ) {
           ^
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/atombios_i2c.c

@@ -35,7 +35,7 @@
 
 static int radeon_process_i2c_ch(struct radeon_i2c_chan *chan,
 				 u8 slave_addr, u8 flags,
-				 u8 *buf, u8 num)
+				 u8 *buf, int num)
 {
 	struct drm_device *dev = chan->dev;
 	struct radeon_device *rdev = dev->dev_private;