staging: fsl-mc: include irqreturn.h as needed

The recent reshuffling of header files left one type definition
missing in some configurations:

In file included from drivers/staging/fsl-dpaa2/ethernet/dpaa2-eth.h:39:0,
                 from drivers/staging/fsl-dpaa2/ethernet/dpaa2-ethtool.c:34:
drivers/staging/fsl-dpaa2/ethernet/../../fsl-mc/include/dpaa2-io.h:86:1: error: unknown type name 'irqreturn_t'; did you mean 'irq_count'?
 irqreturn_t dpaa2_io_irq(struct dpaa2_io *obj);

I have not bisected this to the exact patch that introduced the problem,
but the solution is obvious, we just need to include the header that
contains the type.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>