USB: serial: sierra driver indat_callback fix

USB: serial: sierra driver indat_callback fix

commit b87c6e86dac1bb5222279cc8ff7e09529e1c4ed9 upstream.

A crash has been reported with sierra driver on disconnect with
Ubuntu/Lucid distribution based on kernel-2.6.32.
The cause of the crash was determined as "NULL tty pointer was being
referenced" and the NULL pointer was passed by sierra_indat_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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>