mmc: sdio: improve mmc_io_rw_extended

Improve mmc_io_rw_extended a little:
- using DIV_ROUND_UP achieves the same but is better readable
- simplify code by using sg_set_buf
- simplify one statement by using -= operator

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>