fs_mgr: Clear AvbOps struct.

This ensures that future operations (e.g. function pointers in the
AvbOps struct) added to AvbOps are set to NULL.

Bug: 37709309
Test: Manually tested on UEFI-based bootloader.
Change-Id: If83ea57b7abad94e472768c594efa210e3351a4d
1 file changed