mtd: nand: Fix length bug in ioread16_rep() and iowrite16_rep()

The ioread16_rep() and iowrite16_rep() implementations are U-Boot specific
and have been introduced with the Linux MTD v3.14 sync. While introducing
these functions, the length for the loop has been miscalculated. The ">> 1"
is already present in the caller. So lets remove it in the function.

Tested on omap3_ha.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Scott Wood <scottwood@freescale.com>
Acked-by: Heiko Schocher <hs@denx.de>