usb: usb_bam: Fix concurrency between disconnect and suspend

This commit fixes a crash when disconnecting USB cable and disconnect
sequence finishes after suspend sequence already put the core to LPM.
This fix prevents suspend sequence to enter LPM if disconnect sequence
started and lets disconnect sequence to put the core into LPM.

CRs-fixed: 491064
Change-Id: I65bfce677aae147b9fb790969f41b7ecb5ab9c51
Signed-off-by: Lena Salman <esalman@codeaurora.org>