staging: et131x: Reduce split lines in et131x_rx_dma_memory_alloc()

Modify temporary variables so that split lines can be reduced:

index -> k
fbr_tmp_physaddr -> fbr_physaddr

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>