i40e: Fix set max_tx_rate when it is lower than 1 Mbps

While converting max_tx_rate from bytes to Mbps, this value was set to 0,
if the original value was lower than 125000 bytes (1 Mbps). This would
cause no transmission rate limiting to occur. This happened due to lack of
check of max_tx_rate against the 1 Mbps value for max_tx_rate and the
following division by 125000. Fix this issue by adding a helper
i40e_bw_bytes_to_mbits() which sets max_tx_rate to minimum usable value of
50 Mbps, if its value is less than 1 Mbps, otherwise do the required
conversion by dividing by 125000.

Fixes: 5ecae412

 ("i40e: Refactor VF BW rate limiting")
Signed-off-by: Michal Jaron <michalx.jaron@intel.com>
Signed-off-by: Andrii Staikov <andrii.staikov@intel.com>
Tested-by: Bharathi Sreenivas <bharathi.sreenivas@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>