mtd: nand: omap: fix error-codes returned from omap-elm driver

This patch
 omap-elm.c: replaces -ve integer value returned during errorneous condition,
             with proper error-codes.
 omap-gpmc.c: updates omap-gpmc driver to pass error-codes returned from
             omap-elm driver to upper layers

Signed-off-by: Pekon Gupta <pekon@ti.com>
Reviewed-by: Stefan Roese <sr@denx.de>