fwserial: (coding style) useless "extern" & space

Style-only modifications to make checkpatch.pl --file --strict a bit happier.
Removed useless "extern" in dma_fifo.h ;
Removed one supernumerary space.

Signed-off-by: Dominique van den Broeck <domdevlin@free.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>