hwmon: (nct7904) Rename pwm attributes to match hwmon ABI

pwm attributes have well defined names, which should be used.

Cc: Vadim V. Vlasov <vvlasov@dev.rtsoft.ru>
Cc: stable@vger.kernel.org #v4.1+
Signed-off-by: Guenter Roeck <linux@roeck-us.net>