Miscellaneous fixes and improvements to MMCCAM stack
- Demote the level of several debug messages to CAM_DEBUG_TRACE
- Add detection for SDHC cards that can do 1.8V. No voltage switch sequence is issued yet;
- Don't create a separate LUN for each SDIO function. We need just one to make pass(4) attach;
- Remove obsolete mmc_sdio* files. SDIO functionality will be moved into the separate device that will manage a new sdio(4) bus;
- Terminate probing if got no reply to CMD0;
- Make bcm2835 SDHCI host controller driver compile with 'option MMCCAM'.
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D12109