V4L/DVB: dvb-usb: gp8psk, fix potential null derefernce

Stanse found that in gp8psk_load_bcm4500fw there is missing a check for
return value of kmalloc. Add one and bail out appropriatelly.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>