drm/amdgpu: remove always false comparison in 'amdgpu_atombios_i2c_process_i2c_ch'

Fixes gcc '-Wtype-limits' warning:

drivers/gpu/drm/amd/amdgpu/atombios_i2c.c: In function
‘amdgpu_atombios_i2c_process_i2c_ch’:
drivers/gpu/drm/amd/amdgpu/atombios_i2c.c:79:11: warning: comparison is
always false due to limited range of data type [-Wtype-limits]

'num' is 'u8', so it will never be greater than 'TOM_MAX_HW_I2C_READ',
which is defined as 255. Therefore, the comparison can be removed.

Fixes: d38ceaf9 ("drm/amdgpu: add core driver (v4)")
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>