misc: atmel-ssc: prepare clock only when request

Prepare SSC clock only when request SSC channel, the clock will be
enabled when initialize the SSC.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>