Normally it never needs to wait here at all; waiting is done at the end
of the prior command. When doing a crash dump, the normal interrupt
mechanism isn't used; instead the interrupt handler is called repeatedly
in a polling-like manner. This can subvert hardware-specific drivers
and lead to trying to start a new command while the previous command is
still busy on the bus. Since the SD spec says the longest a card can
take to execute any command is 250ms, use that as a timeout.
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