Serial fixes

[colinleroy]

• Allow changing parity mode after connection is opened
• Fix return value of gp_port_serial_write so that bytes written can be --debug-ged
• Fix inverted logic on parity error checking

The first one is because I will need it to add support for the Quicktake 100 and 150. The two other ones are little bugs :)

[msmeissn]

thanks!