libbinder_driver: never enforce data avail

It would be good to cover this too, but we do have it covered
via binder_parcel_fuzzer now, and this causes a lot of wasted
cycles for our AIDL fuzzers.

Bug: N/A
Test: diffing strace of runs before/after
Change-Id: I743b62272c402c400c5e357adf5538dc5440f2a6