mmc: vub300: Don't use mmc_card_present() when validating for inserted card

The mmc_card_present() function helps the mmc core to track an internal
state of the card device. More importantly, it's not intended to be used by
mmc host drivers to check for an inserted card. Therefore, let's stop using
it and instead rely on checking for a valid pointer to a struct mmc_card,
as it should be good enough.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>