net/usb: remove default in Kconfig for sierra_net driver

The following patch removes the default from the Kconfig entry for sierra_net
driver as recommended.

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