The MMC spec says "It is strongly recommended for hosts to implement
more than 500ms timeout value even if the card indicates the 250ms
maximum busy length."  Even the previous value of 300ms is known to
be insufficient for some cards. So, increase the timeout to 500ms.
Signed-off-by: NYangbo Lu <yangbo.lu@nxp.com>
Reviewed-by: NYork Sun <york.sun@nxp.com>