usb: Remove unused member in struct mos_softc

Spotted by jhibbits in D37812.

Reviewed by: network, glebius
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D38581

(cherry picked from commit 3bafe73affe37de3e94d91d4c1310f1272375970)
(cherry picked from commit 1ab8695033149213e49af0897e2e305649c52817)