drm/amd/display: Remove redundant non-zero and overflow check

[Why]
Unsigned int is guaranteed to be >= 0, and read_channel_reply checks for
overflows. read_channel_reply also returns -1 on error, which is what
dc_link_aux_transfer is expected to return on error.

[How]
Remove the if-statement. Return result of read_channel_reply directly.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Mikita Lipski <Mikita.Lipski@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>