Merge pull request #2 from kaffeemonster/usb_lowlat

Usb lowlat
- Added bit position computation for a given sx channel
- Small typo fix
Added function to read a channel byte
Fixed variable for counting the transferred bits
Smal fixes and bugs
Got sx_get_channel(uint8_t channel) to work
tune sx interrupt handling start to support write

Change the interrupt handler to sqeeze out some instructions.
Enable write support.