USB: serial: sierra driver write path improvements

- Updated Copyright notice with new authors names
 - Version number set to 1.3.6
 - Added a MAX_TRANSFER constant following Greg Kroah-Hartman's
   recommended setting of PAGE_SIZE-512 for USB transfer buffers and
   modified accordingly sierra_write() function.

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