activate_fw.c: Make use of errx() to print error messages to stderr and exit
activate_fw.c: Make use of errx() to print error messages to stderr and exit

This is a more compact way to do it compared to using fprintf and explicitly
exiting in main()

Signed-off-by: Chandni Verma <dad063f8ade782763c74e4953e61405f236df268@linux.vnet.ibm.com>

shenki/powerpc-utils,shenki/powerpc-utils,shenki/powerpc-utils