avoid race between *_probe finishing and an irq

[  849.367069] Unable to handle kernel NULL pointer dereference at virtual address 00000110
...
[  849.911679] [<c0388679>] (pps_event+0x21/0x15c) from [<bf81e23b>] (pps_gmtimer_interrupt+0xb6/0xc8 [pps_gmtimer])
[  849.932654] [<bf81e23b>] (pps_gmtimer_interrupt+0xb6/0xc8 [pps_gmtimer]) from [<c0077577>] (handle_irq_event_percpu+0x4f/0x164)
..
[  850.208559] [<c0257db9>] (pinctrl_select_state+0x19/0x28) from [<bf81e3fd>] (pps_gmtimer_probe+0x1b0/0x2b4 [pps_gmtimer])
[  850.231756] [<bf81e3fd>] (pps_gmtimer_probe+0x1b0/0x2b4 [pps_gmtimer]) from [<c02b848d>] (driver_probe_device+0x51/0x178)
..