staging: wlags49_h2: Fix sparse warning for static declaration

Fix the following sparse warning:
drivers/staging/wlags49_h2/wl_netdev.c:533:6: warning: symbol 'wl_poll'
was not declared. Should it be static?

Signed-off-by: Jelena Bjelja <jelena.bjelja.ing@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>