wimax/i2400m: move I2400M_MAX_MTU enum from netdev.c to i2400m.h

This patch moves I2400M_MAX_MTU enum defined in netdev.c to i2400m.h.
Follow up changes will make use of this value in other location,
thus requiring it to be moved to a global header file i2400m.h.

Signed-off-by: Prasanna S. Panchamukhi <prasannax.s.panchamukhi@intel.com>
Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>