iio: light: vl6180: Drop unused linux/of.h include

Nothing from linux/of.h is used in this driver.

Reviewed-by: Nuno Sa <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/light/vl6180.c

···
 #include <linux/i2c.h>
 #include <linux/mutex.h>
 #include <linux/err.h>
-#include <linux/of.h>
 #include <linux/delay.h>
 #include <linux/util_macros.h>