mx28: fix i.MX28 spi driver

The generic spi flash driver (drivers/mtd/spi/spi_flash.c) uses the
spi low level driver's spi_xfer() function with len=0 to deassert the
SPI flash' chip select. But the i.MX28 spi driver rejects this call
due to len=0.

This patch implements an exception for len=0 with the SPI_XFER_END
flag set. This results in an extra read with the chip select being
deasserted afterwards. There seems to be no way to deassert the signal
by hand.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>