iwlwifi: pcie: make array 'prop' static, shrinks object size

Don't populate the array prop on the stack but instead make it static.
Makes the object code smaller by 30 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  80138	  15382	    576	  96096	  17760	drivers/net/wireless/intel/iwlwifi/pcie/trans.o

After:
   text	   data	    bss	    dec	    hex	filename
  79948	  15542	    576	  96066	  17742	drivers/net/wireless/intel/iwlwifi/pcie/trans.o

(gcc version 8.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>