counter: microchip-tcb-capture: check the correct variable

This should be testing "regmap" instead of "priv->regmap".  The
"priv->regmap" variable is always zero so it's not an error pointer.

Fixes: 106b1041

 ("counter: Add microchip TCB capture counter")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>