mmc: queue: Factor out mmc_queue_alloc_sgs()

In preparation for supporting a queue of requests, factor out
mmc_queue_alloc_sgs().

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Harjani Ritesh <riteshh@codeaurora.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>