[media] media: em28xx: Paranoic stack save

This patch saves 255 bytes of stack on usb_probe() by removing
a char array. In some platforms this is represents a substantial save.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>