mmc: dw_mmc: update kernel-doc comments for dw_mci

cur_slot and num_slots has been removed from struct dw_mci in 42f989c0.
Unfortunately, inline documentation was not updated so far.

Fix @lock field documentation in Locking section.
Move @mrq field of struct dw_mci_slot mention closer to it
description, so no one could miss this slightest detail.

Couple of code style fixes as a bonus.

Signed-off-by: Alexey Roslyakov <alexey.roslyakov@gmail.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>