avr_uart.c: fix to prevent overwriting readonly flags when setting u2x.

	modified:   simavr/sim/avr_uart.c
Merge pull request #33 from bsekisser/uart-u2x-bugfix

avr_uart.c: fix to prevent overwriting readonly flags when setting u2x.
Merge branch 'master' of https://github.com/buserror-uk/simavr into fast-core-testing