iio: mlx90614: Refactor register symbols

The defined registers only make sense when used for accessing RAM. Make
MLX90614_OP_RAM part of the symbol definition to avoid accidental access
to the wrong register.

Signed-off-by: Vianney le Clément de Saint-Marcq <vianney.leclement@essensium.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>