Miscellaneous fixes and improvements to MMCCAM stack

[kibab]

Summary

• 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'.

Test Plan

build kernel with MMCCAM option and without 'device mmc' and 'device mmcsd'