staging: octeon: use defines instead of magic numbers

The ugly magic number 65392 is waiting for CVMX_IPD_MAX_MTU
to appear in the mips tree.

Signed-off-by: Asbjoern Sloth Toennesen <asbjorn@asbjorn.st>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>