o Another round fixes for mmc(4), mmcsd(4) and sdhci(4) regarding

comments, marking unused parameters as such, style(9), whitespace,
etc.

o In the mmc(4) bridges and sdhci(4) (bus) front-ends:

• Remove redundant assignments of the default bus_generic_print_child device method (I've whipped these out of the tree as part of r227843 once, but they keep coming back ...),
• use DEVMETHOD_END,
• use NULL instead of 0 for pointers.

o Trim/adjust includes.