mmc: queue: rename mmc_request function

The name mmc_request is used for both the issue function
and a data structure, which creates conflicts in symbol lookups
in editors. Rename the function to mmc_request_fn.

Signed-off-by: Venkatraman S <svenkatr@ti.com>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>