[IPX]: Correct return type of ipx_map_frame_type().

Casting BE16 to int and back may or may not work. Correct, to be sure.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>