Add timeout logic to sdhci, separate from the timeouts done by the hardware.

If the hardware is not in a good state (like maybe clocks aren't running
because of a configuration glitch) its timeout clock may also not work
correctly, and the next command sent will hang that thread forever.  The
thread in question is usually the one and only thread (at init time) or
a bio queue worker thread whose lockup will eventually lead to the whole
system locking up when it runs out of buffers.

No sd card command should take longer than 250ms.  This new code establishes
a 1-second timeout to allow plenty of safety margin over that.