mmc: core: Cleanup printing of speed mode at card insertion

The current print of the bus speed mode in mmc_add_card() has grown over
the years and is now difficult to parse. Let's clean up the code and also
take the opportunity to properly announce "DDR" for eMMCs as
"high speed DDR", which is according to the eMMC spec.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20240913102836.6144-2-victorshihgli@gmail.com