drivers/fmc: change registration prototype

Permit use of either fmc_device_register_n or fmc_device_register_n_gw
depending on the type of device in use.

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
Tested-by: Pat Riehecky <riehecky@fnal.gov>
Acked-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>