iio: proximity: sx9324: Correct proximity channel resolution

The proximity channels were previously defined with 12 realbits.
However, PROXDIFF is read from RegDiffMsb (0x65) and RegDiffLsb
(0x66). The SX9324 datasheet assigns bits 7:0 of each register to
PROXDIFF and documents it as a signed two's-complement value
(Revision 3, Section 8, Table 8, page 43). In contrast, RegOffsetMsb
explicitly marks bits 7:6 as reserved. Thus, PROXDIFF is a 16-bit
signed value.

With realbits = 12, sx_common_read_proximity() uses bit 11 as the
sign bit in sign_extend32(), causing samples outside the 12-bit
signed range to wrap into the [-2048, 2047] range.

Correct the realbits value to 16 to accurately reflect the hardware.

Tested on an SX9324-based device: a phase 0 DIFF readback of 0x7fff
was reported as -1 before this change and as 32767 afterward.

Fixes: 4c18a890df ("iio:proximity:sx9324: Add SX9324 support")
Cc: stable@vger.kernel.org
Signed-off-by: Zhang Jie <zhangjie14@huaqin.corp-partner.google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>

drivers/iio/proximity/sx9324.c

@@ -222,7 +222,7 @@ static const struct iio_chan_spec_ext_info sx9324_channel_ext_info[] = {
 	.scan_index = idx,					 \
 	.scan_type = {						 \
 		.sign = 's',					 \
-		.realbits = 12,					 \
+		.realbits = 16,					 \
 		.storagebits = 16,				 \
 		.endianness = IIO_BE,				 \
 	},							 \