MFC r326924-r326925

r326924:
Fix debugging output, fallout from something like s/read/readctl/g
while renaming variables in a previous change.

r326925:
Do not attempt to refill the TX fifo if there is no data left to transfer.

A comment in bcm_bsc_fill_tx_fifo() even lists sc_totlen > 0 as a
precondition for calling the routine. I apparently forgot to make the
code do what my comment said.