net: phy: mxl-gpy: enhance delay time required by loopback disable function

GPY2xx devices need 3 seconds to fully switch out of loopback mode
before it can safely re-enter loopback mode. Implement timeout mechanism
to guarantee 3 seconds waited before re-enter loopback mode.

Signed-off-by: Xu Liang <lxu@maxlinear.com>
Signed-off-by: David S. Miller <davem@davemloft.net>