usb: dwc3: Remove tasklet bottom half handler

There is a possibility of tasklet bottom half handler racing
with dwc3_msm_suspend(). As a result before disabling the dwc3
irq, dwc3 interrupts are fired and once the suspend routine
disables the clocks bottom half handler gets a chance to run
and tries to access dwc3 register resulting into un-clocked
register access. Since dwc3 interrupt is already running in
threaded irq context, interrupt can be directly handled in
same context and avoid this race.

Change-Id: Ife9e165f6aa2112c1440819d659b97b5502a3f07
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>