drivers: i2c_nrfx_twim: refactor concat check logic

The condition of being either the last message or not concatenable to
the next message is the inverse of the condition of committing to
concatenate the next message.  The latter is arguably easier to
understand.  Reverse the logic, document the conditions, and simplify
the early continue.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>