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]