soc: fsl: dpio: fix memory leak of a struct qbman on error exit path

Currently the error check for a null reg leaks a struct qbman
that was allocated earlier. Fix this by kfree'ing p on the error exit
path.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>