drivers: sdhc: imx_usdhc: add explicit fallthrough to I/O timing setup

DDR50/DDR52 modes should use PINCTRL_STATE_SLOW (50MHz), so the lack of a
break statement after enabling DDR mode is expected. Add an explicit
__fallthrough to resolve the issue flagged by coverity scan

Fixes #65324

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>