[USB CDC] Only re-enable IRQ if PRIMASK was 0 before disabling IRQ

[sandeepmistry]

As discussed by @matthijskooijman in #65 (comment):

Also, it might be good to let accept() use the same conditional interrupts() call (in a separate commit, of course)?

Also, incorporated feedback from #65 (comment)

To emphasize the non-portability (and reduce confusion for people reading the code), it might be better to even just use __disable_irq() and __enable_irq() directly?

[matthijskooijman]

Looks good! I would remove the { } from the if for conciseness, but that's a matter of style/taste :-)

[cmaglie]

Rebased and merged.