char: xillybus: use devm_add_action_or_reset

If devm_add_action() fails we are explicitly calling dma_unmap_single(),
pci_unmap_single() and kfree(). Lets use the helper
devm_add_action_or_reset() and return directly in case of error, as we
know that the cleanup function has been already called by the helper if
there was any error. At that same time remove the variable rc which
becomes unused now.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>