xdp: Simplify __bpf_tx_xdp_map()

The explicit error checking is not needed. Simply return the error
instead.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20191219061006.21980-9-bjorn.topel@gmail.com