Use properly aligned buffer in usb_alloc

The PA adress must be gathered from an aligned VA,
not the RAW pointer to the memory space.

Reviewed by: hselasky
Submitted by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by: Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4140