net: phy: micrel: add an option to disable gigabit for the KSZ9031

The environment variable "disable_giga" can now be used to disable
1000baseTx on the Micrel's KSZ9031.

Signed-off-by: Sebastien Bourdelin <sebastien.bourdelin@savoirfairelinux.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>