dme1737: Keep index within pwm_config[]

The static code scanner "Parfait" reported this because pwm_config is
only 3 bytes - pwm_config[3] is out of range.

Since this code path is never called with ix == 3 (the device has no PWM4
output) this doesn't change anything in practice.  But to encourage
testing with Parfait, lets make the warning go away...

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>