x/sys/unix: add missing const (MTDFILEMODE) from <mtd/mtd-user.h>

[lhl2617]

What did you expect to see?

#46063 added most of these consts; however, one const--MTDFILEMODE--did not make it due to an overly strict regex in mkerrors.sh:

...
		$2 ~ /^MTD_/ ||
...

This can be trivially fixed by removing the underscore.

Apologies--I did not catch this in my CL.

[gopherbot]

Change https://golang.org/cl/323391 mentions this issue: unix: add missing const (MTDFILEMODE) from <mtd/mtd-user.h>