bnxt_en: Fix invalid max channel parameter in ethtool -l.

When there is only 1 MSI-X vector or in INTA mode, tx and rx pre-set
max channel parameters are shown incorrectly in ethtool -l.  With only 1
vector, bnxt_get_max_rings() will return -ENOMEM.  bnxt_get_channels
should check this return value, and set max_rx/max_tx to 0 if it is
non-zero.

Signed-off-by: Satish Baddipadige <sbaddipa@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>