net: fec: remove .ndo_poll_controller to avoid deadlocks

In the Linux kernel, the following vulnerability has been resolved:

net: fec: remove .ndo_poll_controller to avoid deadlocks

There is a deadlock issue found in sungem driver, please refer to the

commit ac0a230f719b ("eth: sungem: remove .ndo_poll_controller to avoid

deadlocks"). The root cause of the issue is that netpoll is in atomic

context and disable_irq() is called by .ndo_poll_controller interface

of sungem driver, however, disable_irq() might sleep. After analyzing

the implementation of fec_poll_controller(), the fec driver should have

the same issue. Due to the fec driver uses NAPI for TX completions, the

.ndo_poll_controller is unnecessary to be implemented in the fec driver,

so fec_poll_controller() can be safely removed.