netx/xc: add a few "const"s to fix compiler warnings

the fixed warnings are:

	arch/arm/mach-netx/xc.c: In function ‘xc_request_firmware’:
	arch/arm/mach-netx/xc.c:152: warning: assignment discards qualifiers from pointer target type
	arch/arm/mach-netx/xc.c:162: warning: assignment discards qualifiers from pointer target type

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>