iwlwifi: correct math in elapsed_jiffies

w/o the first change: if end == start you get MAX_JIFFY_OFFSET which
isn't what you want...

For the latter I think to be technically correct you need the +1 to
account for the jiffy between MAX_JIFFY_OFFSET and 0

(hmm w/ the 2nd change the first isn't strictly needed... ah well)

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>