drm/amdgpu: fix a signedness bug in __verify_ras_table_checksum()

If amdgpu_eeprom_read() returns a negative error code then the error
handling checks:

	if (res < buf_size) {

The problem is that "buf_size" is a u32 so negative values are type
promoted to a high positive values and the condition is false.  Fix
this by changing the type of "buf_size" to int.

Fixes: 63d4c081 ("drm/amdgpu: Optimize EEPROM RAS table I/O")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>