usb: fix undefined behavior caused by zero-length array

The use of zero-length array causes undefined behavior.
Replace zero-length array with flexible-array member.

Bug: 174330770
Test: build pass and boot to home
Change-Id: I080ed1555d5b60075f6a86aa0ac15427529f7a1b
Signed-off-by: Jimmy Hu <hhhuuu@google.com>
Signed-off-by: Puma Hsu <pumahsu@google.com>