Restore original MDC speed control register value after MAC reset, if it
wasn't default

Since vte_reset changes register value to MDCSC_DEFAULT value, which may not
be the original value, thus causing some phy registers read failures.
Restoring VTE_MDCSC value to original after reset solves the link state
flapping issue.

Thanks to jhb ("the code looks ok") for his review.
Reviewed by: jhb
Obtained from: NetBSD via Andrius V
Differential Revision: https://reviews.freebsd.org/D34956