intel_s1000_crb: messenger.py: Make endian_swap() static

Doesn't use 'self'. Fixes this pylint warning:

    boards/xtensa/intel_s1000_crb/support/messenger.py:50:4: R0201:
    Method could be a function (no-self-use)

If this function is meant to be internal to messenger.py, then a better
option than @staticmethod might be to turn it into a regular function.

Fixing pylint warnings for a CI check.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>