usb: gadget: add check of existing swfi_latency before updating request

Before calling pm_qos_update_request a check for swfi_latency was added
to prevent calling it with un-initialized request (in platforms that do
not support this feature yet)

Change-Id: I6f05c7c6aad4deafc048d2988415b56a731e0ad6
Signed-off-by: Ofir Cohen <ofirc@codeaurora.org>