hwmon: (it87) Refactor attributes creation and removal

There is a lot of code redundancy in the creation of the fan and
pwm attributes. Move these attributes to arrays so that the code can
be simplified.

This in turns makes the attributes removal code larger, so move it to
a separate function that can be called in both the standard removal
case and the error path during probing.

Signed-off-by: Jean Delvare <khali@linux-fr.org>