qlnxr: Properly initialize the Linux device structure

The driver needs to provide a LinuxKPI device structure to register
itself with the IB subsystem. It was erroneously using a copy of its
FreeBSD device structure for this purpose.

Use linux_pci_attach_device() instead, following the example of the
Chelsio iwarp driver. Also ensure that we don't leak the faked device
during detach.

Reviewed by: hselasky
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29595