mmc: queue: declare mmc_alloc_sg as static

Fix the sparse warning "drivers/mmc/card/queue.c:111:20: warning:
symbol 'mmc_alloc_sg' was not declared. Should it be static?"

Signed-off-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>