mtd, omap: fix case NAND_OMAP_GPMC_PREFETCH not defined

mtd, omap: fix case NAND_OMAP_GPMC_PREFETCH not defined

The patch c316f577b4b72d1a1aeb559e9b3fad20808f1ffd breaks
siemens boards because prefetch mode is not enabled.
I assume it breaks other boards as well that don't use
prefetch.

This patch sets read_buf to nand_read_buf if
NAND_OMAP_GPMC_PREFETCH is not defined.

Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
CC: Daniel Mack <zonque@gmail.com>
CC: Guido Martínez <guido@vanguardiasur.com.ar>
CC: Tom Rini <trini@ti.com>
CC: Heiko Schocher <hs@denx.de>