usb: dwc2: Change reading of current frame number flow.

The current frame_number is read from core for both
device and host modes. Reading of the current frame
number needs to be performed ASAP due to IRQ latency's.
This is why, it is moved to common interrupt handler.

Accordingly updated dwc2_gadget_target_frame_elapsed()
function which uses stored frame_number instead of
reading frame number.

In cases when target frame value is incremented
the frame_number is required to read again.

Signed-off-by: Artur Petrosyan <arturp@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>