hwmon: (pmbus/ltc2978) Code cleanup

Use u16 instead of int to store cached limit attributes.
This reduces allocated data size per driver instance by 48 bytes.
Use defines for the number of pages supported by individual chips.
Use ARRAY_SIZE for loops to initialize array variables, and
initialize all variables in the same code block.

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