ucc_geth: Fix NULL pointer dereference in uec_get_ethtool_stats()

In commit 3e73fc9a ("ucc_geth: Fix IO
memory (un)mapping code") I fixed ug_regs IO memory leak by properly
freeing the allocated memory. But ethtool_stats() callback doesn't
check for ug_regs being NULL, and that causes following oops if
'ethtool -S' is executed on a closed eth device:

  Unable to handle kernel paging request for data at address 0x00000180
  Faulting instruction address: 0xc0208228
  Oops: Kernel access of bad area, sig: 11 [#1]
  ...
  NIP [c0208228] uec_get_ethtool_stats+0x38/0x140
  LR [c02559a0] ethtool_get_stats+0xf8/0x23c
  Call Trace:
  [ef87bcd0] [c025597c] ethtool_get_stats+0xd4/0x23c (unreliable)
  [ef87bd00] [c025706c] dev_ethtool+0xfe8/0x11bc
  [ef87be00] [c0252b5c] dev_ioctl+0x454/0x6a8
  ...
  ---[ end trace 77fff1162a9586b0 ]---
  Segmentation fault

This patch fixes the issue.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>