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

change sequence of FM slave port close to be called after
LPASS port close (remove hw_free),use watermark level of 8
for FM slave ports and don't reset the watermark to 0
upon port disable.

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