MFC: r340495

MFC: r340495

- Restore setting the clock for devices which support the default/legacy
  transfer mode only (lost with r321385). [1]
- Similarly, don't try to set the power class on MMC devices that comply
  to version 4.0 of the system specification but are operated in default/
  legacy transfer or 1-bit bus mode as no power class is specified for
  these cases. Trying to set a power class nevertheless resulted in an -
  albeit harmless - error message.

PR:	231713 [1]

git-svn-id: svn://svn.freebsd.org/base/stable/10@340741 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f