USB: msm72k_udc: Don't handle asynchronous interrupt

USB enter low power mode after detecting the wall charger. When
wall-charger is disconnected, USB hardware generate asynchronous interrupt
for notifying B-Session Valid low condition. OTG and UDC share the same
interrupt line. The OTG interrupt handler is called first and it disable
the IRQ and schedule the resume work. As the interrupt is received,
the kernel will invoke all shared interrupt handlers though the IRQ is
disabled by the first handler.

The UDC interrupt handler is accessing USB registers before clocks are
turned on. Avoid unclocked access by ignoring the asynchronous interrupt.

CRs-Fixed: 442012
Change-Id: I58c6a6e192ce274ab265b5d7c8d95ad84dc55efd
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>