ARM: ux500: alter MSP registration to return a device pointer

Alter the db8500_add_msp_i2s() calls to return a struct
platform_device * pointer, not an integer. We nee the pointer
to obtain a pinctrl handle.

Cc: Ola Lilja <ola.o.lilja@stericsson.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>