[TG3]: Eliminate the TG3_FLAG_5701_REG_WRITE_BUG flag.

This patch removes the use of the TG3_FLAG_5701_REG_WRITE_BUG flag.
It's logic is only used to set a function pointer and thus the
logic can be collapsed and the flag removed.

[ Comment tidy by Christoph Hellwig. -DaveM ]

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>