net: rfkill: Fix sparse warning in rfkill-regulator.c

'rfkill_regulator_ops' is used only in this file. Hence make it static.
Silences the following warning:
net/rfkill/rfkill-regulator.c:54:19: warning:
symbol 'rfkill_regulator_ops' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>