mmc: block: break out mmc_blk_rw_start_new()

As a step toward breaking apart the very complex function
mmc_blk_issue_rw_rq() we break out the code to start a new
request.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>