drivers/nble: Fix passing correct length to poll_out()

The RPC code is not net_buf aware so it will not update the net_buf
length when encoding data into it. We have to therefore do it
ourselves.

Change-Id: I7d7d3c09007a2a9bce822625607dbf7470a7ea0b
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>