bnxt_en: Fix ethtool -l|-L inconsistent channel counts.

The existing code is inconsistent in reporting and accepting the combined
channel count.  bnxt_get_channels() reports maximum combined as the
maximum rx count.  bnxt_set_channels() accepts combined count that
cannot be bigger than max rx or max tx.

For example, if max rx = 2 and max tx = 1, we report max supported
combined to be 2.  But if the user tries to set combined to 2, it will
fail because 2 is bigger than max tx which is 1.

Fix the code to be consistent.  Max allowed combined = max(max_rx, max_tx).
We will accept a combined channel count <= max(max_rx, max_tx).

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