wimax/i2400m: modify i2400m_tx_fifo_push() to check for head room space in the TX FIFO [v1]

This fixes i2400m_tx_fifo_push(); the check for having enough
space in the TX FIFO's tail was obscure and broken in certain
corner cases. The new check works in all cases and is way
clearer. Please refer the documentation in the code for details.

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