usb: gadget: f_mass_storage: Allow USB LPM upon ep disable

Upon cable disconnect, there is a possibility of USB LPM
before file-storage thread run to completion. This causes
ep dequeue to fail and results into thread to block for
ever. Prevent this situation by incrementing power usage
count on gadget parent upon ep enable and decrement it
upon ep disable and allow USB LPM after that.

Change-Id: Ic3633320de6ebef912bdaeac305b3dd0df3eb8e0
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>