RootCanal: Fix behaviour of Read on closed Socket

Reading from a closed socket should be returning 0, instead
it returns EBADHDL as the file descriptor is replaced by -1
after closing. This causes aborts down the line when RootCanal
fulfills a scheduled Read from a closed HCI socket.

Test: Manual
Change-Id: I637078285de26c48a887b4956397b66df2b5c7bd