gianfar: Add support for byte queue limits.

Add support for byte queue limits (BQL), based on the similar
modifications made to intel/igb/igb_main.c from Eric Dumazet
in commit bdbc0631
	"igb: Add support for byte queue limits."

A local variable for tx_queue->qindex was introduced in
gfar_clean_tx_ring, since it is now used often enough to warrant it,
and it cleans up the readability somewhat as well.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>