Allow early termination of 50 consecutive beacons.

This value is the recommended one by the 12xx's system/RF team,
and tests show that power consumption is improved as expected.
Reported-by: Ruthy Zaphir <ruthyz@ti.com>
Tested-by: Danil Shalumov <danils@ti.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>