hwmon: (pmbus/mp2975) Replace home made version of __assign_bit()

The newly introduced SWAP() macro is quite generic by naming, but
moreover it's a repetition of the existing __assign_bit().
With this applied, add a missing bits.h (via now required bitops.h).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240325120952.3019767-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>