mmc: block: rename rqc and req

In the function mmc_blk_issue_rw_rq() the new request coming in
from the block layer is called "rqc" and the old request that
was potentially just returned back from the asynchronous
mechanism is called "req".

This is really confusing when trying to analyze and understand
the code, it becomes a perceptual nightmare to me. Maybe others
have better parserheads but it is not working for me.

Rename "rqc" to "new_req" and "req" to "old_req" to reflect what
is semantically going on into the syntax.

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