staging: fsl-mc/dpio: Fix cast truncate warning

Sparse reports the following warning:
drivers/staging/fsl-mc/include/dpaa2-fd.h:421:30:
warning: cast truncates bits from constant value (ffff7fff becomes 7fff)

Fix this by explicitly masking the value with 0xffff.

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