eth: fbnic: hwmon: Add completion infrastructure for firmware requests

eth: fbnic: hwmon: Add completion infrastructure for firmware requests

Add infrastructure to support firmware request/response handling with
completions. Add a completion structure to track message state including
message type for matching, completion for waiting for response, and
result for error propagation. Use existing spinlock to protect the writes.
The data from the various response types will be added to the "union u"
by subsequent commits.

Signed-off-by: Sanman Pradhan <sanman.p211993@gmail.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Link: https://patch.msgid.link/20250114000705.2081288-2-sanman.p211993@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>