bnxt_en: Fix VNIC clearing logic for 57500 chips.

During device shutdown, the VNIC clearing sequence needs to be modified
to free the VNIC first before freeing the RSS contexts.  The current
code is doing the reverse and we can get mis-directed RX completions
to CP ring ID 0 when the RSS contexts are freed and zeroed.  The clearing
of RSS contexts is not required with the new sequence.

Refactor the VNIC clearing logic into a new function bnxt_clear_vnic()
and do the chip specific VNIC clearing sequence.

Fixes: 7b3af4f7 ("bnxt_en: Add RSS support for 57500 chips.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>