net/usb: initiate sync sequence in sierra_net.c driver

The following patch adds the initiation of the sync sequence to
"sierra_net_bind()". If this step is omitted, the modem will never sync up
with the host and it will not be possible to establish a data connection.

Signed-off-by: Elina Pasheva <epasheva@sierrawireless.com>
Signed-off-by: Rory Filer <rfiler@sierrawireless.com>
Tested-by: Elina Pasheva <epasheva@sierrawireless.com>
Signed-off-by: David S. Miller <davem@davemloft.net>