MIPS: asm/errno.h: switch to asm-generic/errno.h

This fixes several warnings like

In file included from ./u-boot/include/linux/mtd/mtd.h:13:0,
                 from env_onenand.c:37:
./u-boot/build/vct_platinumavc_onenand_small/include2/asm/errno.h:52:0: warning: "ENOMSG" redefined [enabled by default]

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>