soc: fsl: qbman: Fix return value on success

On error the function was meant to return -ERRNO.  This also fixes
compile warning:

  drivers/soc/fsl/qbman/bman.c:640:6: warning: variable 'err' set but not used [-Wunused-but-set-variable]

Fixes: 0505d00c ("soc/fsl/qbman: Cleanup buffer pools if BMan was initialized prior to bootup")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Li Yang <leoyang.li@nxp.com>