hwmon: Add PEC attribute support to hardware monitoring core

hwmon: Add PEC attribute support to hardware monitoring core

Several hardware monitoring chips optionally support Packet Error Checking
(PEC). For some chips, PEC support can be enabled simply by setting
I2C_CLIENT_PEC in the i2c client data structure. Others require chip
specific code to enable or disable PEC support.

Introduce hwmon_chip_pec and HWMON_C_PEC to simplify adding configurable
PEC support for hardware monitoring drivers. A driver can set HWMON_C_PEC
in its chip information data to indicate PEC support. If a chip requires
chip specific code to enable or disable PEC support, the driver only needs
to implement support for the hwmon_chip_pec attribute to its write
function.

Packet Error Checking is only supported for SMBus devices. HWMON_C_PEC
must therefore only be set by a driver if the parent device is an I2C
device. Attempts to set HWMON_C_PEC on any other device type is not
supported and rejected.

The code calls i2c_check_functionality() to check if PEC is supported
by the I2C/SMBus controller. This function is only available if CONFIG_I2C
is enabled and reachable. For this reason, the added code needs to depend
on reachability of CONFIG_I2C.

Cc: Radu Sabau <radu.sabau@analog.com>
Acked-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>