bnxt: do not busy-poll when link is down

When busy polling while a link is down (during a link-flap test), TX
timeouts were observed as well as the following messages in the ring
buffer:

bnxt_en 0008:01:00.2 enP8p1s0f2d2: Resp cmpl intr err msg: 0x51
bnxt_en 0008:01:00.2 enP8p1s0f2d2: hwrm_ring_free tx failed. rc:-1
bnxt_en 0008:01:00.2 enP8p1s0f2d2: Resp cmpl intr err msg: 0x51
bnxt_en 0008:01:00.2 enP8p1s0f2d2: hwrm_ring_free rx failed. rc:-1

These were resolved by checking for link status and returning if link
was not up.

Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Tested-by: Rob Miller <rob.miller@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>