bnxt_en: Check max_tx_scheduler_inputs value from firmware.

When checking for the maximum pre-set TX channels for ethtool -l, we
need to check the current max_tx_scheduler_inputs parameter from firmware.
This parameter specifies the max input for the internal QoS nodes currently
available to this function.  The function's TX rings will be capped by this
parameter.  By adding this logic, we provide a more accurate pre-set max
TX channels to the user.

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