Fix XMEGA USB prescaler calculation

The USB prescaler calculation for the CLK.USBCTRL register is changed to
give valid results and set the prescaler correctly.

Signed-off-by: Bert van Hall <bert.vanhall@avionic-design.de>
Merge pull request #32 from The-42/fix-xmega-usb-prescaler

Fix XMEGA USB prescaler calculation (thanks to Bert van Hall).
Minor code style fixes, documentation improvements.