e1000e: fix a missing check for return value

The change is based on the issue found by Kangjie Lu <kjlu@umn.edu> where
we not checking the return value of a register read/write which could result
in a NULL pointer dereference if the read/write fails.

Since we are only trying to disable the far-end loopback, if the read
and write of register fails, we do not want to bail out of the function.
We just want to log that it failed to disable and continue on.

CC: Sasha Neftin <sasha.neftin@intel.com>
CC: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>