btfm: set watermark level to 3 for wcn3990 slimbus FM slave ports

Without this, we hear silence or delay in sound during FM playback in
following use cases -
1. Switching between FM and media applications like YouTube
2. Changing volume during FM FTM

Change-Id: I51072ab70e6bc34e1ed10222ee289d987ed9ffbf
Signed-off-by: Rupesh Tatiya <rtatiya@codeaurora.org>