staging: fsl-mc: dpio: remove foolish -Werror Makefile addtion

With the zillion different compilers out there, never use -Werror,
otherwise your code will end up breaking the build for odd reasons.

Like now, if this driver is enabled, it breaks the build due to a
function that could be marked static.  So it's obvious no one is even
paying attention to this driver :(

Cc: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>