nimble/ll: Fix access address computation
nimble/ll: Fix access address computation

Core 5.0, Vol 6, Part B, section 2.1.2:
    The Access Address shall be a 32-bit value. Each time it needs a
    new Access Address, the Link Layer shall generate a new random value
    that meets the following requirements:
      (...)
      * It shall have a minimum of two transitions in the most
        significant six bits.

Current implementation of the above only checks if there are no
transitions in 6 msb so it can create invalid access address.

Transitions can be easily calculated using xor with the same value
shifted - the result will have '1' at each position with transition so
then it's just a matter of counting them.

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