ASoC: wcd9xxx: request bandgap in slow mode

Since bandgap mode change requires clock block disablement when bandgap
get and clock put which are followed by clock get and bandgap put happen,
two clock get or put happen unnecessary.
Reqeust bandgap mode in slow mode to avoid unnecessary temporary clock
source changes.  Also regardless of this change, resource manager always
requests bandgap in slow mode as fast mode request is meaningless.

Change-Id: I6ff52d004cfab6372373b7405f66d8777b3396fd
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>