usb: qmp: phy: Make sure QMP PHY write completed

Add explicit write memory barrier before turning off QMP PHY clock to
make sure previous POWER DOWN operation (in disconnect case) or
putting PHY into autonomous mode (in suspend case) is completed.
Similar way add write memory barrier after performing POWER UP
operation (in connect case) or disabling PHY autonomous mode (in
resume case).

Change-Id: I91bb098506eb0c7d5dd6248fad71b8bb9186ea5a
Signed-off-by: Mayank Rana <mrana@codeaurora.org>