[media] fix a warning on avr32 arch

X-Patchwork-Delegate: m.chehab@samsung.com
on avr32 arch, those warnings happen:
	drivers/media/firewire/firedtv-fw.c: In function 'node_update':
	drivers/media/firewire/firedtv-fw.c:329: warning: comparison is always true due to limited range of data type

In this particular case, the signal is desired, as the isochannel
var can be initalized with -1 inside the driver.

So, change the type to s8, to avoid issues on archs where char
is unsigned.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Reviewed-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>