eth: fbnic: rename fbnic_fw_clear_cmpl to fbnic_mbx_clear_cmpl

eth: fbnic: rename fbnic_fw_clear_cmpl to fbnic_mbx_clear_cmpl

fbnic_fw_clear_cmpl() does the inverse of fbnic_mbx_set_cmpl().
It removes the completion from the mailbox table.
It also calls fbnic_mbx_set_cmpl_slot() internally.
It should have fbnic_mbx prefix, not fbnic_fw.
I'm not very clear on what the distinction is between the two
prefixes but the matching "set" and "clear" functions should
use the same prefix.

While at it move the "clear" function closer to the "set".

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250624142834.3275164-6-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>