nand, gpmc: fix reading after switching ecc

The omap_gpmc allows switching ecc at runtime. Since
the NAND_SUBPAGE_READ flag is only set, it is kept when
switching to hw ecc, which is not correct. This leads to
calling chip->ecc.read_subpage which is not a valid
pointer. Therefore clear the flag when switching ecc so
reading in hw mode works again.

Cc: Scott Wood <scottwood@freescale.com>
Cc: Pekon Gupta <pekon@ti.com>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>