Improve busy-wait loop during switch phy access in e6000sw

Hitherto implementation of PHY polling resulted in a risk of an
endless loop and very high occupation of the SMI bus. Improve the
operation by limiting the polling tries and adding sleepable
pause.

Submitted by: Marcin Wojtas <mw@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: loos
Differential revision: https://reviews.freebsd.org/D10713