soc: fsl: dpio: remove redundant pointer 'priv'

Pointer 'priv' is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
variable 'priv' set but not used [-Wunused-but-set-variable]

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