Bluetooth: Controller: Fix nRF51x advertising channel PDU regression

Fix nRF51x series advertising channel PDU S1 byte
configuration regression, that causes corrupted advertising
PDUs being reported to Host, by adding the missing bits_s1
local variable initialization.

Regression introduced in commit 1dcbe73c ("Bluetooth:
controller: radio: add setting of S1 byte in radio packet").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>