px4io: added isr_debug() call

this allows for debug from ISR handlers
px4io: fixed array reference bug
px4io: added some more debug output
px4io: cope with missed RX interrupt
px4io: more debug output calls
px4io: support SET_DEBUG call
px4io: fixed cpp error
px4io: change uploader buffer size to 4
px4io: added px4io debug command and ioctl
px4io: fixed initialisation error
px4io: fixed setting _connected flag
px4io: fixed uninitialised return
px4io: added some debug output to io_reg_get
px4io: fixed mixer load
px4io: debug_level needs to be volatile
px4io: a few more debug lines
px4io: spam console less at debug level 1
px4io: reset I2C bus is we lockup for more than 100ms

this happens every 20 minutes or so. It also dumps I2C registers, so
we can see what needs fixing. Here is a typical register dump:

RESETTING line=288
CR1   0x00000401  CR2   0x00000b18
OAR1  0x00000034  OAR2  0x00000000
CCR   0x0000c003  TRISE 0x00000008
SR1   0x00000044  SR2   0x00000002
px4io: better way to handle lockups

much less delay to IO processing
docs: fixed typo in comments on clock rate