ath9k: simplify ATH_EP_RND

Remove the embedded branch to make the ATH_EP_RND macro a little
clearer.  The new version also generates better code, saving 24
bytes of text:

   text	   data	    bss	    dec	    hex	filename
  87858	   1641	     24	  89523	  15db3	ath9k_orig.ko
  87834	   1641	     24	  89499	  15d9b	ath9k_new.ko

Although neither version handles negative values particularly well,
the lone caller clamps all negative values to zero anyway.  I have
verified that the results are the same for the range of possible
positive rssi values.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>