bnxt_en: Reserve rings in bnxt_set_channels() if device is down.

The current code does not reserve rings during ethtool -L when the device
is down.  The rings will be reserved when the device is later opened.

Change it to reserve rings during ethtool -L when the device is down.
This provides a better guarantee that the device open will be successful
when the rings are reserved ahead of time.

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