nimble/controller: Fix handling incorrect LL opcode
nimble/controller: Fix handling incorrect LL opcode

According to BT specification v5.0 Vol.6 Part B, 2.4.2

If an LL Control PDU is:
* not supported
* not used
* invalid i.e. set to value that is Reserved for Future use

or CtrlData is invalid, the Link Layer shall respond with an
LL_UNKNOWN_RSP PDU.

This closes #633

X-Original-Commit: f5b3bf68445d645866366bd8c25ce2031215770f

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