mmc: omap_hsmmc: Check if MMC slot name is passed in pdata

mmc: omap_hsmmc: Check if MMC slot name is passed in pdata

The legacy user space for n900 relies on the MMC slot names.
Let's check if those are passed in pdata and use them.

As this makes the DT booting compatible with legacy booting,
we should be able to start dropping omap3 legacy booting
support in v4.8.

Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: linux-mmc@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>
Tested-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>