[MTD] [NAND] OMAP2: add retry after read timeout

Very occasionally, (about one in a million) read operations are
ongoing after the timeout has expired.  So, retry three times
while the ongoing bit remains set.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>