usb: dwc3-msm: Prevent suspend during pwr_event_handler

While a pwr event IRQ can arrive asynchronously, the handler
takes care to only call dwc3_pwr_event_handler() when not in
LPM mode since the core clock needs to be on to access the
status registers. However, this doesn't account for concurrent
runtime PM occurring which could still result in an unclocked
access if suspend completes before pwr_event_handler() does.
Prevent a premature suspend by temporarily incrementing the
runtime PM count at the beginning of pwr_event_handler() and
calling put() when done.

Change-Id: I0d0d26445c5ff0f7c7a3fc51862e4fa8a1ff7d6e
Signed-off-by: Jack Pham <jackp@codeaurora.org>