Staging: wlags49_h2: remove unused function prototype

With today linux-next I got a compile error in staging/wlags49_h2 driver
due an unused function prototype that use a data type (event_callback_args_t)
that doesn't exist anymore in the pcmcia code.

Current patch solves the issue removing the function prototype.

Signed-off-by: Javier Martinez Canillas <martinez.javier@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>