mmc: block: allow get_card_status() to return error status

If the MMC_SEND_STATUS command is not successful, we should not return
a zero status word, but instead allow the caller to know positively
that an error occurred.

Convert the open-coded get_card_status() to use the helper function,
and provide definitions for the card state field.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Chris Ball <cjb@laptop.org>