Minor cleanups in mmc_xpt.c

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