spi: omap3: Fix timeout handling

The timeout value is never reset during the transfer. This means that when
transferring more data we eventually trigger the timeout.

This was reported on the mailing list:
"Spansion SPI flash read timeout with AM335x"

Signed-off-by: David Dueck <davidcdueck@googlemail.com>
CC: Tom Rini <trini@konsulko.com>
CC: Stefan Roese <sr@denx.de>
CC: Andy Pont <andy.pont@sdcsystems.com>
Tested-by: David Dueck <davidcdueck@googlemail.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>