tty: resolve some sierra breakage

The various merges into the sierra driver inadvertently undid
commit 212b8f0c

 by Elina Pasheva
<epasheva@sierrawireless.com>. Put it back so the OBEX port works again.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>