staging: fsl-mc-bus: fix build warning

error could be unitialized when it is used as a return value in
fsl_mc_device_add().  So fix up the warning by properly setting it.

Cc: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>