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>

drivers/iio/adc/mcp3422.c

@@ -88,10 +88,10 @@ static const int mcp3422_sample_rates[4] = {
 
 /* sample rates to sign extension table */
 static const int mcp3422_sign_extend[4] = {
-	[MCP3422_SRATE_240] = 12,
-	[MCP3422_SRATE_60] = 14,
-	[MCP3422_SRATE_15] = 16,
-	[MCP3422_SRATE_3] = 18 };
+	[MCP3422_SRATE_240] = 11,
+	[MCP3422_SRATE_60] = 13,
+	[MCP3422_SRATE_15] = 15,
+	[MCP3422_SRATE_3] = 17 };
 
 /* Client data (each client gets its own) */
 struct mcp3422 {