btfm: Close 3990 FM tx ports first and keep overflow bit always set

Close data source ports i.e. wcn3990 tx ports before data sink ports i.e.
AFE for FM tx case. Also, do not set wcn3990 overflow/underflow registers
to 0 during channel disable path. This causes audio loss if audio focus
is lost during FM playback.

Change-Id: Id6c8ee145bf7feb8681851499ba36b708667b4ef
Signed-off-by: Rupesh Tatiya <rtatiya@codeaurora.org>