[Trac #251] Timeout calculation causes divide by zero on stopped SD card.

[Zarro Boogs per Child]

#251: Timeout calculation causes divide by zero on stopped SD card.
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 Reporter:  dwmw2   |       Owner:  blizzard
     Type:  defect  |      Status:  new     
 Priority:  normal  |   Milestone:  BTest-1 
Component:  distro  |    Keywords:          
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 When we're resetting SD we set card->host->ios.clock to zero. If requests
 are handled in that state, we divide zero in mmc_set_data_timeout().

 The attached patch possibly isn't the best fix -- we probably ought to be
 stopping the block queue altogether when we reset the host. But it at
 least avoids the oops.

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Ticket URL: <http://dev.laptop.org/ticket/251>
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