pcie-gadget-spear: Add "platform:" prefix for platform modalias

Since 43cc71ee (platform: prefix MODALIAS
with "platform:"), the platform modalias is prefixed with "platform:".

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Pratyush Anand <pratyush.anand@st.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>