nvme: Only reset once on attach.

The FreeBSD nvme driver has reset the nvme controller twice on attach to
address a theoretical issue assuring the hardware is in a known
state. However, exierence has shown the second reset is unnecessary and
increases the time to boot. Eliminate the second reset. Should there be
a situation when you need a second reset (for buggy or at least somewhat
out of the mainstream hardware), the hardware option NVME_2X_RESET will
restore the old behavior. Document this in nvme(4).

If there's any trouble at all with this, I'll add a sysctl tunable to
control it.

Sponsored by: Netflix
Reviewed by: cperciva, mav
Differential Revision: https://reviews.freebsd.org/D32241