ucc_geth: Fix netdev watchdog triggering on link changes

Since commit 864fdf88

 ("ucc_geth:
Fix hangs after switching from full to half duplex") ucc_geth driver
disables the controller during MAC configuration changes. Though,
disabling the controller might take quite awhile, and so the netdev
watchdog might get upset:

 NETDEV WATCHDOG: eth2 (ucc_geth): transmit queue 0 timed out
 ------------[ cut here ]------------
 Badness at c02729a8 [verbose debug info unavailable]
 NIP: c02729a8 LR: c02729a8 CTR: c01b6088
 REGS: c0451c40 TRAP: 0700   Not tainted  (2.6.32-trunk-8360e)
 [...]
 NIP [c02729a8] dev_watchdog+0x280/0x290
 LR [c02729a8] dev_watchdog+0x280/0x290
 Call Trace:
 [c0451cf0] [c02729a8] dev_watchdog+0x280/0x290 (unreliable)
 [c0451d50] [c00377c4] run_timer_softirq+0x164/0x224
 [c0451da0] [c0032a38] __do_softirq+0xb8/0x13c
 [c0451df0] [c00065cc] do_softirq+0xa0/0xac
 [c0451e00] [c003280c] irq_exit+0x7c/0x9c
 [c0451e10] [c00640c4] __ipipe_sync_stage+0x248/0x24c
 [...]

This patch fixes the issue by detaching the netdev during the
time we change the configuration.

Reported-by: Lennart Sorensen <lsorense@csclub.uwaterloo.ca>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Tested-by: Lennart Sorensen <lsorense@csclub.uwaterloo.ca>
Cc: Stable <stable@vger.kernel.org> [2.6.32]
Signed-off-by: David S. Miller <davem@davemloft.net>