USB:dwc3-msm: Don't schedule work if pm_qos_latency is zero

Currently driver is scheduling pm_qos_work again even though
pm_qos_latency is not passed from dts file. Add a check for
pm_qos_latency and don't schedule work if pm_qos_latency value
is zero. Also remove use of static variable for last_irq_count and
add the variable for the same in dwc3 structure for the case where
multi DWC3 usages are used.

Change-Id: I55e1e3a7d48fbea0a421802aae176ac57a48869f
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>