counter: ti-ecap-capture: fix IS_ERR() vs NULL check

The devm_counter_alloc() function returns NULL on error.  It doesn't
return error pointers.

Fixes: 4e2f42aa

 ("counter: ti-ecap-capture: capture driver support for ECAP")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Julien Panis <jpanis@baylibre.com>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/Y0bUbZvfDJHBG9C6@kili/
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>