Limited Ethernet speed to 10/100 Mbps

Due to a problem in U-MoBo schematics, the board does not work against Gigabit Ethernet switches, when mounting an i.MX6 SoM.

This change to Linux driver for Micrel KSZ9031 prevent it from advertising its Gigabit capabilities, allowing the board to work with Gigabit switches at reduced speed (10/100 MBps).

Since the hardware does not work at all when connected to Gigabit switches, then I think that this change MUST be merged into the original u-mobo/linux-imx repository; the only other option is to modify the hardware, but we cannot do that on GitHub ;) and anyway it won't fix the boards already produced.