regmap: mmio: convert some error returns to BUG()

Some of the error conditions detected by regmap_mmio_*() are pure internal
errors, rather than user-/client-triggerable conditions. Convert these to
BUG().

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>