pwm: imx: Signedness bug in imx_pwm_get_state()

"ret" only holds zero and negative error codes.  It needs to be signed
for the error handling to work.

Fixes: 9f4c8f96

 ("pwm: imx: Add ipg clock operation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>