sun7i:axp209: Fix "BUG: scheduling while atomic" messages

On poweroff, the A20 kernel complains about the schedule_timeout
calls made by the i2c driver called from the various
axp_set_bits/axp_clr_bits/axp_read/axp_write calls in axp_power_off.

Setting preempt_count() to zero prevents this by making believe the
kernel is preemptable even though we're in the middle of an atomic
operation (shutdown).  This has no impact on any other code, as
axp_power_off doesn't return. In the unlikely event the system
doesn't power off, the kernel exits anyway.

Signed-off-by: Patrick Wood <patrickhwood@gmail.com>