usb: dwc3: Improve lfps handshake interop by reducing u3 exit time

SNPS star 9001276244:LFPS Handshake Interoperability Issues suggests
to reduce U3 exit handshake timer to from 8us to 300ns to increase
interoperability with hosts/devices which supports USB 3.1 gen2
speeds.

Change-Id: I68c6f5f8a09337c5c3c70c2f0b6940efcba0dab8
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>