Staging: winbond: mds_f: deleted extern function

Deleted an extern function that wasn't used in this driver.

Signed-off-by: Iker Pedrosa <ikerpedrosam@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>