drm/amd/display: Replace uint8_t with u8 for dp_hdmi_dongle_signature_str

The string dp_hdmi_dongle_signature_str already uses u8 but the string
dp_hdmi_dongle_signature_str does not. Just replace uint8_t with u8 for
dp_hdmi_dongle_signature_str.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>