Improve error/debug messages in sdhci.c

[bz]

Summary

Improve error/debug messages in sdhci.c

When starting a command also print the opcode and flags.
More consitently print flags as hex.
Use slot_printf rather than printf in one case.

Sponsored by: The FreeBSD Foundation
MFC After: 2 weeks

[marius]

Looks good to me, it would be great if you could expand "Resp" to "Response" and use "result" instead of "err" for consistency while at it, though.