mmc: core: Re-factor code for sending CID

Instead of having the caller to check for SPI mode, let's leave that to
internals of mmc_send_cid(). In this way the code gets cleaner and it
becomes clear what is specific to SPI and non-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>