hwmon: (f75375s) pwmX_mode sysfs files writable for f75375 variant

Fix value check in set_pwm_mode(). Instead of checking for chip variant there,
make pwmX_mode sysfs nodes only writable on f75375 variant.

Signed-off-by: Riku Voipio <riku.voipio@movial.fi>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>