MYNEWT-687: Reduce encryption scratchpad size on nrf51
MYNEWT-687: Reduce encryption scratchpad size on nrf51

The nrf51 only required the maximum encrypted packet length (plus 16)
for its encryption scratchpad register. The code was using the max
PDU length of 251. The nrf51 can only support 27 byte encrypted frames.

X-Original-Commit: e7934fafce21fd27138fd9e15373ce44e30da682

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble