usb: dwc3-msm: Clear pending power events before setting irq mask

commit 9c07090e4f54 ("usb: dwc3-msm: Set in_lpm only after turning off
clocks") disables power event irq before setting OUT L2/P3 event irq mask
and enables later. Upon cable disconnect dwc3_otg_start_peripheral()
resumes SS phy which results into OUT P3 irq as soon as OUT P3 mask is
set upon controller suspend. Since power event irq is disabled, interrupt
remains pending until power event irq gets enabled, this causes controller
to exit from low power mode right after it goes to LPM.

CRs-Fixed: 844372
Change-Id: I138650e945639f82ce8d3c42c1336f3205bd2530
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>