linux-phytec-ti

sound: oss: fix build warning

while building with allyesconfig it was giving a build warning about
unused variable. declare the variable only if the driver is built as a
module.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>