iio: adc: mcp3911: use correct formula for AD conversion

stable inclusion
from stable-v5.10.142
commit 30fd0e23e373ba9edb4cc5bcc85bead94bfd4e22
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6CSFH

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=30fd0e23e373ba9edb4cc5bcc85bead94bfd4e22



--------------------------------

commit 9e2238e3 upstream.

The ADC conversion is actually not rail-to-rail but with a factor 1.5.
Make use of this factor when calculating actual voltage.

Fixes: 3a89b289 ("iio: adc: add support for mcp3911")
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220722130726.7627-4-marcus.folkesson@gmail.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>
Reviewed-by: Zheng Zengkai <zhengzengkai@huawei.com>