txtzyme.c, usbserial.c: Improve compatibility with newer gcc-avr

I had compilation failures with my avr-gcc related to util/delay.h:

   /usr/local/CrossPack-AVR-20121207/lib/gcc/avr/4.6.2/../../../../avr/include/util/delay.h:164:28: error: __builtin_avr_delay_cycles expects an integer constant.

and usbserial.c:

    usb_serial.c:147:24: error: variable 'device_descriptor' must be const in order to be put into read-only section by means of '__attribute__((progmem))'

defining the  __DELAY_BACKWARD_COMPATIBLE__ as per util/delay.h and setting the const attributes on the descriptors in usb_serial.h fixes these problems.