staging: et131x: Don't handle rx/tx packets when changing mtu

There's no need to handle any rx/tx interrupts in the middle of an mtu
change, so don't.

After this change, receive and transmit interrupts are only handled in
one place, which paves the way to using NAPI.

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