regmap: slimbus: allow register offsets up to 16 bits

As per SLIMBus specs Value Elements and Information Elements
address map ranges from 0x000 - 0xFFF.

So allow register addresses up to 16 bits

Fixes: 7d6f7fb0 ("regmap: add SLIMbus support")
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>