Always return MMC errors from mmc_handle_reply()

There are two ways to propagate the error in MMCCAM:

• Using cmd.error which is set by the peripheral driver;
• Using CCB status which is... also set by the driver.

The problem is that those two error conditions don't necessarily match.
This leads to the confusion when handling the MMC reply. So enforce the consistency
by panicking if request is marked as completed successfully but MMC-level error
is present (this hints to the programming error).

Reviewed by: manu
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D26925