Update pwm cached control registers before writing pwm enable

Update pwm cached control registers before writing pwm enable

If pwmX_enable is written prior to displaying any attributes,
t87_update_pwm_ctrl() is never called, and the cached pwm registers
are never read before written. This results in bad values written
into various registers, and can result in operational failures.

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