firewire: rename source files

The source files of firewire-core, firewire-ohci, firewire-sbp2, i.e.
 "drivers/firewire/fw-*.c"
are renamed to
 "drivers/firewire/core-*.c",
 "drivers/firewire/ohci.c",
 "drivers/firewire/sbp2.c".

The old fw- prefix was redundant to the directory name.  The new core-
prefix distinguishes the files according to which driver they belong to.

This change comes a little late, but still before further firewire
drivers are added as anticipated RSN.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>