lpfc: use time_after()

To be future-proof and for better readability the time comparisons
are modified to use time_after() instead of plain, error-prone math.

Signed-off-by: Manuel Schölling <manuel.schoelling@gmx.de>
Reviewed-by: James Smart <james.smart@emulex.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>