firewire: nosy: fix build when CONFIG_FIREWIRE=N

drivers/firewire/nosy* is a stand-alone driver that does not depend on
CONFIG_FIREWIRE.  Hence let make descend into drivers/firewire/ also
if that option is off.

The stand-alone driver drivers/ieee1394/init_ohci1394_dma*  will soon be
moved into drivers/firewire/ too and will require the same makefile fix.

Side effect:
As mentioned in https://bugzilla.novell.com/show_bug.cgi?id=586172#c24
this influences the order in which either firewire-ohci or ohci1394 is
going to be bound to an OHCI-1394 controller in case of a modular build
of both drivers if no modprobe blacklist entries are configured.
However, a user of such a setup cannot expect deterministic behavior
anyway.  The Kconfig help and the migration guide at
ieee1394.wiki.kernel.org recommend blacklist entries when a dual
IEEE 1394 stack build is being used.  (The coexistence period of the two
stacks is planned to end soon.)

Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>