USB: serial: sierra driver version change to 1.3.8

Updated sierra driver version from 1.3.7 to 1.3.8 now that the autosuspend
capabilities were added to the driver.

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