kernel/prism: fix LOS alarm initial state

The Marvell GPON kernel driver does not properly clear the LOS alarm
after the board first boots. The Marvell kernel code will set the LOS
alarm correctly if there is a real loss of signal alarm. So changing
the initial value seems to correct the problem.

Addresses https://b/31153124

Change-Id: If5ecad6fd7b410f2cca96e421c1359a3c9827662
(cherry picked from commit 78450a9b1c8cb540d4c784571fff5adcf97bd670)
1 file changed