staging: et131x: Remove unnecessary PHY register write

The PHY registers are now being controlled from the connected phydev,
so there shouldn't be any reason for the et131x code to perform any
extra setup. Removing the interrupt setup code, and register defines
that are now unused.

On testing, no changes in behaviour were experienced.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>