usb: composite: 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 two functions 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>