[BNX2]: Add missing wait in bnx2_init_5709_context().

For correctness, we need to wait for the MEM_INIT bit to be cleared
in the BNX2_CTX_COMMAND register before proceeding.

[Added return -EBUSY when the MEM_INIT bit doesn't clear, suggested
by Jeff Garzik.]

Signed-off-by: Michael Chan <mchan@broadcom.com>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>