wimax/i2400m: fix BUILD_BUG_ON() to use the maximum message size constant [v1]

The older method of computing the maximum PDU size relied
on a method that doesn't work when we prop the maximum
number of payloads up to the physical limit, and thus we kill
the whole computation and just verify that the constants are
congruent.

Signed-off-by: Prasanna S. Panchamukhi <prasannax.s.panchamukhi@intel.com>