ena: Prevent LLQ initialization when membar isn't exposed

The ena_com_config_dev_mode() function performs many LLQ related
calculations and sends an admin command to configure LLQ in the device.

All the LLQ related operations are unnecessary if the driver fails to
find LLQ memory bar.

Move LLQ memory bar allocation to separate helper function
ena_map_llq_mem_bar and execute this function before LLQ configuration.
If the LLQ memory bar cannot be allocated, then LLQ configuration is
skipped.

Obtained from: Semihalf
MFC after: 2 weeks
Sponsored by: Amazon, Inc.