bug fix, No blind USB irq enable

[dfries]

I have a program that's receiving data over USB serial and consuming that data as timers expire (two interrupt routines). I'm having data corruption, hangs, and other odd problems. I disabled interrupts in my routines, but I'm calling the usb_serial routines and they unconditionally enable interrupts. Here are corrected versions of the usb routines that will only enable interrupts if interrupts were previously enabled. This fixed the problems I was seeing.