pwm: lp3943: Drop unused i2c include

lp3943 is a platform driver that doesn't use any symbol provided in
<linux/i2c.h>. So drop the include.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>