sbus: Auto-load openprom module when device opened.

The openprom module is missing the char-major-10-139 alias that would
cause it to be auto-loaded when a device of that type is opened.  This
patch adds the alias.

Signed-off-by: Scott James Remnant <scott@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>