bcm2835-mmc: Honor return value of mmc_of_parse()

bcm2835_mmc_probe() ignores errors returned by mmc_of_parse() and in
particular ignores -EPROBE_DEFER, which may be returned if the power
sequencing driver configured in the devicetree is compiled as a module.

The user-visible result is that access to the SDIO device fails because
its power sequencing requirements have not been observed.  Fix it.

Signed-off-by: Lukas Wunner <lukas@wunner.de>