mmc: sd: fix crash with sd card

Change ID I1333e1d4330393e446ba48a9474c83295744c050
added a call to the bus specific runtime_idle
callback. This callback is only implemented for mmc
thereby causing a crash when other types of memory
(e.g. sd card) are used.
This patch only calls the callback if it is implemented.

Change-Id: Ie26b1b52fdcc0f4d5e3638801e28e965fae0cbeb
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>