mmc: core: Clarify code for sending CSD

To make the code more consistent and to increase readability, add an
mmc_spi_send_csd() function, which gets called from mmc_send_csd() in case
of SPI mode.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>