hwmon: (pmbus) Simplify remove functions

Since devm_kzalloc() is now used to allocate driver memory, the client
driver remove function has no purpose other than to call pmbus_do_remove().
This means we can get rid of it by redefining pmbus_do_remove() to use the
same prototype, and pointing to it directly.

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