usb.c contains three independent parts with no common part.

Split it.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Drop usb.o from Makefile to fix build]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/75712b54bf9cb85ab10e47cd2772cd2a098ca895.1692199324.git.christophe.leroy@csgroup.eu