disk: Fix usdhc driver out-of-bounds array access

The usdhc driver was incorrectly post-decrementing a do/while loop
iterator, causing an extra iteration of the loop and an out-of-bounds
array access. Change the iterator to decrement within the loop instead.

Tested samples/subsys/fs/fat_fs on the mimxrt1050_evk board.

Coverity-CID: 203403

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>