callback().

This patch modifies sierra_indat_callback() function to check for NULL
tty structure pointer. This modification prevents a crash from happening
when the device is disconnected.

This patch fixes the bug reported in Launchpad:
  https://bugs.launchpad.net/ubuntu/+source/linux/+bug/511157

Signed-off-by: Elina Pasheva <epasheva@sierrawireless.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

p