Bluetooth: Controller: Fix observer filter_policy field size

Coverity analysis discovered that observer filter policy
field was 1 bit, whereas valid range for extended scanner
filter policy feature implemented in controller is 0 to 3.
Increase the bit field size from 1 to 2.

Change-Id: Id4b2e354961dfb3b45f72fa4e0ab18de7425bbb5
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>