* Work around for serial buffer over run

PronterFace sends a lot of M105 commands.  During long operations such
as UBL's G29 P1, G29 P2, G29 P4 and G26 this can over run the serial
buffer.   This results (very often) in a M1 (actually a M1M105) ending
up in the command queue.

Until we figure out a better way to resolve this issue, this will keep
the UBL commands from experiencing bogus commands at thier completion.