Bluetooth: Remove bogus condition for setting IRK for an identity

Just like we set the address for an identity without depending on the
BT_DEV_READY flag, we should do the same for the IRK. Otherwise we
risk getting an all-zeroes IRK. Remove the condition and always set
the IRK value whenever CONFIG_BT_PRIVACY is enabled.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>