net: phy: leds: Clear phy_num_led_triggers on failure to avoid crash

phy_attach_direct() ignores errors returned by
phy_led_triggers_register(). I think that's OK, as LED triggers can be
considered a non-critical feature.

However, this causes problems later:
  - phy_led_trigger_change_speed() will access the array
    phy_device.phy_led_triggers, which has been freed in the error path
    of phy_led_triggers_register(), which may lead to a crash.

  - phy_led_triggers_unregister() will access the same array, leading to
    crashes during s2ram or poweroff, like:

	Unable to handle kernel NULL pointer dereference at virtual address
	00000000
	...
	[<c04116d4>] (__list_del_entry_valid) from [<c05e8948>] (led_trigger_unregister+0x34/0xcc)
	[<c05e8948>] (led_trigger_unregister) from [<c05336c4>] (phy_led_triggers_unregister+0x28/0x34)
	[<c05336c4>] (phy_led_triggers_unregister) from [<c0531d44>] (phy_detach+0x30/0x74)
	[<c0531d44>] (phy_detach) from [<c0538bdc>] (sh_eth_close+0x64/0x9c)
	[<c0538bdc>] (sh_eth_close) from [<c04d4ce0>] (dpm_run_callback+0x48/0xc8)

    or:

	list_del corruption. prev->next should be dede6540, but was 2e323931
	------------[ cut here ]------------
	kernel BUG at lib/list_debug.c:52!
	...
	[<c02f6d70>] (__list_del_entry_valid) from [<c0425168>] (led_trigger_unregister+0x34/0xcc)
	[<c0425168>] (led_trigger_unregister) from [<c03a05a0>] (phy_led_triggers_unregister+0x28/0x34)
	[<c03a05a0>] (phy_led_triggers_unregister) from [<c039ec04>] (phy_detach+0x30/0x74)
	[<c039ec04>] (phy_detach) from [<c03a4fc0>] (sh_eth_close+0x6c/0xa4)
	[<c03a4fc0>] (sh_eth_close) from [<c0483234>] (__dev_close_many+0xac/0xd0)

To fix this, clear phy_device.phy_num_led_triggers in the error path of
phy_led_triggers_register() fails.

Note that the "No phy led trigger registered for speed" message will
still be printed on link speed changes, which is a good cue that
something went wrong with the LED triggers.

Fixes: 2e0bc452 ("net: phy: leds: add support for led triggers on phy link state change")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>