Minor cleanups in mmc_xpt.c

• Downgrade some CAM debug messages from _INFO to _DEBUG level;
• Add KASSERT for the case when we suspect incorrect CAM SIM initialization (using cam_sim_alloc() instead of cam_sim_alloc_dev());
• Use waiting version of xpt_alloc_ccb(), we are not in hurry;
• With the waiting version we cannot get NULL return, so remove the NULL check;
• In some csses, the name of mmcprobe_done has been written as mmc_probedone();
• Send AC_LOST_DEVICE if we, well, lost the device;
• Misc style(9) fixes.

Reviewed by: manu
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D25843