intel10g: Adapt receive/can_receive to not annoy LuaJIT.

This is based on Alex Gall's implementation on the mailing list:

  https://groups.google.com/d/msg/snabb-devel/JFFxbBOrG8k/Aek4VSsl5JwJ
Merge pull request #208 from lukego/intel10g-opt-fix

intel10g: Adapt receive/can_receive to not annoy LuaJIT.