usb(4): Stop checking for failures from malloc(M_WAITOK).

Handle the fact that parts of usb(4) can be compiled into the boot
loader, where M_WAITOK does not guarantee a successful allocation.

PR: 240545
Submitted by: Andrew Reiter <arr@watson.org> (original version)
Reviewed by: hselasky
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25706