n64: use pr_fmt to avoid duplicate string

Instead of repeating the n64cart string all over the module use pr_fmt
macro and remove the duplicate string. Also, replace and with or in the
one of the error message.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Lauri Kasanen <cand@gmx.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>