target/nds32: fix type of magic number

target/nds32: fix type of magic number

The macro NDS32_COMMON_MAGIC was cast-ed to int to avoid compile
time error for comparison type mismatch while comparing it with
the field common_magic.
This is incorrect because the macro value is a 32 bit unsigned
value; better changing the type of the field common_magic to keep
the unsigned value.

Issue identified by checkpatch script from Linux kernel v5.1 using
the command

	find src/ -type f -exec ./tools/scripts/checkpatch.pl \
	-q --types TYPECAST_INT_CONSTANT -f {} \;

Change-Id: Ib5924b6cecdffe70ab5c78d3b30a9c8e4deb7c7b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5193
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>