usb: dwc3: gadget: Prevent tx fifo resize before set config

If device took longer than expected time queuing zlp in status
stage host issues bus reset. As a result tx fifio resize happens
at status stage of first control transfer after bus reset. This
causes null pointer dereference for config pointer as device is
not in configured state yet. Hence clear the resize_fifos flag
upon bus reset to avoid this situation. Also add NULL check in
dwc3_gadget_resize_tx_fifos().

CRs-Fixed: 747688
Change-Id: Icd50eaa15f75c1b0bd7e2f8db5550ad54af47ef8
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Mayank Rana <mrana@codeaurora.org>