lpfc:Make the function lpfc_sli4_mbox_completions_pending static in order to comply with function prototype

This makes the function lpfc_sli4_mbox_completion's definition
static now in order to comply with its prototype being also
declared as static too.
Signed-off-by: NNicholas Krause <xerofoify@gmail.com>
Signed-off-by: NJames Smart <james.smart@avagotech.com>
Reviewed-by: NSebastian Herbszt <herbszt@gmx.de>
Reviewed-by: NHannes Reinecke <hare@suse.com>
Signed-off-by: NJames Bottomley <JBottomley@Odin.com>