x25_asy.c: silence compiler warning

Commit 11b0cc3a

 ("x25_asy: Fix ref count
rule violation") introduced the warning

  drivers/net/wan/x25_asy.c: In function `x25_asy_open_tty':
  drivers/net/wan/x25_asy.c:557: warning: unused variable `ld'

Signed-off-by: S.Çağlar Onur <caglar@pardus.org.tr>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>