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

drivers/net/wireless/iwlwifi/iwl-helpers.h

@@ -246,10 +246,10 @@ static inline int iwl_check_bits(unsigned long field, unsigned long mask)
 static inline unsigned long elapsed_jiffies(unsigned long start,
 					    unsigned long end)
 {
-	if (end > start)
+	if (end >= start)
 		return end - start;
 
-	return end + (MAX_JIFFY_OFFSET - start);
+	return end + (MAX_JIFFY_OFFSET - start) + 1;
 }
 
 static inline u8 iwl_get_dma_hi_address(dma_addr_t addr)