usb: dwc2: fix possible alignment issues

Since upgrading to gcc9, warnings are issued:
"taking address of packed member of ‘...’ may result in an unaligned
pointer value"

Fix this by converting dwc2_fifo_read to use unaligned access since packed
structures may be on an unaligned address, depending on USB hardware.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>