mmc_spi: Fix unterminated of_match_table

commit 2ffe8c5f ("of: refactor
of_modalias_node() and remove explicit match table"), introduced
an unterminated of_match_table, which may cause kernel to oops.

This patch fixes the issue by adding an empty device ID.

Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>