Avoid AXI bus issues due to a MAC reset on imx6sx and imx7.

When the FEC is connected to the AXI bus (indicated by AVB flag), a
MAC reset while a bus transaction is pending can hang the bus.
Instead of resetting, turn off the ENABLE bit, which allows the
hardware to complete any in-progress transfers (appending a bad CRC
to any partial packet) and release the AXI bus. This could probably
be done unconditionally for all hardware variants, but that hasn't
been tested.

PR: 222634
Submitted by: sebastian.huber@embedded-brains.de