No fixed value for USB power current

This should save some cycles inside ISR Handler.

Change the way ISRs are stored. Store now in a list on a ‘first come
first served basis’, the first interrupt that gets attached will be
served first. This improves the speed of the ISR calling from about 6us
to 2us for a single interrupt on extint14

See #200

Solve #199

Fix #223

See https://github.com/arduino/Arduino/issues/4525

PWM frequency 732 Hz

The RXC interrupt was being disabled when using bit to disable the DRE
interrupt.