m68k: fix compiler warning by properly inlining flat_set_persistent()

This patch removes the following warning:
fs/binfmt_flat.c:752: warning: unused variable 'persistent'.
There is neither functionality change, nor extra code generated.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>