SD/MMC: check the card status during erase operation

SD/MMC: check the card status during erase operation

Use the function 'mmc_send_status' to check the card status.
only when the card is ready, driver can send the next erase command
to the card, otherwise, the erase will failed:
=> mmc erase 0 1
MMC erase: dev # 0, block # 0, count 1 ... 1 blocks erase: OK
=> mmc erase 0 2
MMC erase: dev # 0, block # 0, count 2 ... mmc erase failed
1 blocks erase: ERROR
=> mmc erase 0 4
MMC erase: dev # 0, block # 0, count 4 ... mmc erase failed
1 blocks erase: ERROR

Signed-off-by: Jerry Huang <Chang-Ming.Huang@freescale.com>
CC: Andy Fleming <afleming@gmail.com>
CC: Marek Vasut <marex@denx.de>
Signed-off-by: Andy Fleming <afleming@freescale.com>