net: cpsw: do not register cpts twice

commit f280e89a

 (drivers: net: cpsw: fix for cpsw crash when build as modules)
moved cpts_register()/cpts_unregister() to  ndo_open()/ndo_stop(), but failed
to remove cpts_register in cpsw_probe() which leads to a double registration
and the following debug object splat.

[   18.991902] ODEBUG: init active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x2c
[   19.082249] [<c0059e80>] (init_timer_key) from [<c04965d4>] (cpts_register+0x1f0/0x2c4)
[   19.090642] [<c04965d4>] (cpts_register) from [<c04931dc>] (cpsw_ndo_open+0x780/0x81c)
[   19.098948] [<c04931dc>] (cpsw_ndo_open) from [<c0599c2c>] (__dev_open+0xb4/0x118)

Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>