82xx: FCC: Fixing a bug causing to FCC port lock-up (second try)

82xx: FCC: Fixing a bug causing to FCC port lock-up (second try)

This is an additional patch to the one already submitted recently.
The previous patch was not complete, and the FCC port lock-up scenario
has been reproduced in lab.
I had an opportunity to check the current patch in lab and the FCC
port lock no longer freezes, while the previous patch still locks-up the
FCC port.
The current patch fixes a pointer arithmetic bug (second bug in the same
line), which leads FCC port lock-up during underrun/collision handling.
Within the tx_startup() function in mac-fcc.c, the address of last BD is
not calculated correctly. As a result of wrong calculation of the last BD
address, the next transmitted BD may be set to an area out of the transmit
BD ring. This actually causes to port lock-up and it is not recoverable.

Signed-off-by: Martin Roth <martin.roth@motorolasolutions.com>
Signed-off-by: David S. Miller <davem@davemloft.net>