net: rfkill: Do not ignore errors from regulator_enable()

Function regulator_enable() may return an error that has to be checked.
This patch changes function rfkill_regulator_set_block() so that it checks
for the return code.  Also, rfkill_data->reg_enabled is set to 'true' only
if there is no error.

This fixes the following compilation warning:

net/rfkill/rfkill-regulator.c:43:20: warning: ignoring return value of 'regulator_enable', declared with attribute warn_unused_result [-Wunused-result]

Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>