Use the new cam_sim_alloc_dev function to properly initialize SIM

Using cam_sim_alloc_dev() allows to properly set sim_dev field so that
sdiob(4) can attach to the CAM device that represents SDIO card.
The same change for SDHCI driver happened in r348800.

Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D22192