Fix XMEGA USB prescaler calculation

The XMEGA USB prescaler calculation for the CLK.USBCTRL register is way off for at least the atxmega128b1/3 (tested), atxmega32c4 and atxmega256a3bu (datasheets). I assume the USB module is quite the same across all XMEGA devices if implemented, so indeed the same prescaler calculation is needed. However, according to the datasheets (e.g. [XMEGA B Manual](http://www.atmel.com/Images/Atmel-8291-8-and-16-bit-AVR-Microcontrollers-XMEGA-B_Manual.pdf), p. 88) the prescaler bits are defined as:

USBPSDIV[2:0] | Prescaler
------------ | -------------
000 | 1
001 | 2
010 | 4
011 | 8
100 | 16
101 | 32
110 | (reserved)
111 | (reserved)

While for a prescaler of 1 used for full speed USB with a 48 MHz PLL the previous code was correct, for pretty much any other combination like low speed USB with 6 MHz from the same PLL, it wasn't. For some combinations even the reserved bits 6 and 7 in that register were written, and that can't be right, can it ^^

This commit fixes the calculation and has been confirmed to work correctly with an atxmega128b1.