ARM: OMAP2+: GPMC: fix device size setup

following statement can only change device size from 8-bit(0) to 16-bit(1),
but not vice versa:

regval |= GPMC_CONFIG1_DEVICESIZE(wval);

so as this field has 1 reserved bit, that could be used in future,
just clear both bits and then OR with the desired value

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>