mmc: block: make function mmc_cqe_issue_type static

The function mmc_cqe_issue_type is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
drivers/mmc/core/queue.c:62:21: warning: symbol 'mmc_cqe_issue_type'
was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>