firewire: don't use extern on public symbols

Make firewire entry points not 'extern':

drivers/firewire/fw-device.c:160:25: warning: function 'fw_device_get' with external linkage has definition
drivers/firewire/fw-device.c:167:13: warning: function 'fw_device_put' with external linkage has definition

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>