mmc: cqhci: introduce get_trans_desc_offset()

The same calculation to get transfer descriptor offset is already used
at 3 different locations. Let's create a new helper to simplify code.

Signed-off-by: Yue Hu <huyue2@yulong.com>
Link: https://lore.kernel.org/r/20210521034432.2321-1-zbestahu@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>