i2c-explorer: Added i2cgetdata

Using the atomic methods we use the following sequence to read register
15 from the DAC:

> i2cstart
> i2csendaddr 0x4a
> i2csenddata 0x0f
> i2cstart
> i2csendaddr 0x4a 1
> i2cgetdata
[stop is automatic for reads]

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>