max8903: store pointer to pdata instead of copying it.

Stores pointer to pdata because it easily allows pdata to reference
either platform data or in the future device tree data.

Signed-off-by: Chris Lapa <chris@lapa.com.au>

Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>