Increase timeout in Atheros HAL

It turned out, that some models of the Atheros PCIe
adapters (e.g. AR983x family) may fail to attach
due to insufficient timeout value.

Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: adrian
Differential revision: https://reviews.freebsd.org/D10903