btfm: Correct the slimbus turn on/off sequence for FM ports

Correct FM on/off sequence with following changes ->
- FM slave port close is called after LPASS port close (remove hw_free)
- Use the watermark level of 8 for FM slave ports
- Do not reset the watermark to 0 upon port disable

Change-Id: Ie785715c2b92dae6b46b346bebd45e044f54ea53
Signed-off-by: Rupesh Tatiya <rtatiya@codeaurora.org>