cc3200/mods/pybi2c: Make readfnom_mem_into/writeto_mem return None.

This aligns the I2C class to match the standard machine.I2C API.

Note that this is a (small) breaking change to the existing cc3200 API.
The original API just returned the size of the input buffer so there's no
information lost by this change.  To update scripts users should just use
the size of the buffer passed to these functions to get the number of bytes
that are read/written.