iio: Fix sign extension table in mcp3422 driver

the index argument to sign_extend32() gives the bit position (from 0)
to the sign bit

so e.g. if the measurement has 16-bit resolution, we need to pass 15;
a measurement of 0x8000 should be reported as -32768, not 32768

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>