extcon: Add missing notifier head init for the blocking notifiers

This is a fatal bug on RT since rwsems need to be initialized with a value
other than zero, so the missing initialization causes the blocking
notifier's internal rwsem to hang forever. Fix it by adding the missing
initialization.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Change-Id: Ibe35a990bd4c0b93efb2a1fb295a60a2fa454bf0
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>