MTD: mxc_nand: make bit fields unsigned to please sparse

This fixes:

	arch/arm/plat-mxc/include/mach/mxc_nand.h:25:14: error: dubious one-bit signed bitfield
	arch/arm/plat-mxc/include/mach/mxc_nand.h:26:17: error: dubious one-bit signed bitfield

While at it make width unsigned, too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>