USB: serial: sierra driver performance improvements

- Version number set to 1.3.4
 - Increased the number of input/output URBs for improved performance
   (numbers based on an measurement study triggered by a user request).
   We performed the testing using a network simulator that provided full
   speeds in the uplink and downlink directions and this combination of
   URBs provided the best throughput.

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