ssbi: Comment the use of udelay()

The ssbi driver uses a busywait loop to read its status register.  Add
a comment explaining the timing of the device itself so that future
developers can better understand this delay, and possibly diagnose any
problems.

Signed-off-by: David Brown <davidb@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>