adbd: respond to device-to-host control transfers.

Failing to write to the endpoint results in subsequent reads on ep0 to
fail with EL2HLT, so do an empty write to fulfill the transfer.

Bug: http://b/131867920
Test: manually tested with modified auto client
Change-Id: If2eec162ca4b31a9974c7bd5802be51cee9e2708
Merged-In: If2eec162ca4b31a9974c7bd5802be51cee9e2708
(cherry-picked from internal commit af086e9a)