qlnxe: Advertise the IFCAP_HWSTATS capability

The hardware can count statistics and the driver has already retrieved
them via qlnx_get_counter().

Advertise the IFCAP_HWSTATS capability to avoid the net stack from
double counting IFCOUNTER_IBYTES.

Reviewed by: kbowling
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51451

(cherry picked from commit 386960a8805edc6d5ac6bb6215ad102a83314549)