efi_selftest: Receive the packets until the receive buffer is empty

Repeatedly receive the packets until the receive buffer is empty.
If the buffer is empty, EFI_SIMPLE_NETWORK_PROTOCOL::Receive()
returns EFI_NOT_READY. We don't need to use the wait_for_event()
every time.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>