ssb: make array pwr_info_offset static const, makes object smaller

Don't populate the array pwr_info_offset on the stack but instead make it
static const. Makes the object code smaller by 207 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  26066	   3000	     64	  29130	   71ca	drivers/ssb/pci.o

After:
   text	   data	    bss	    dec	    hex	filename
  25763	   3096	     64	  28923	   70fb	drivers/ssb/pci.o

(gcc version 9.2.1, amd64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Michael Büsch <m@bues.ch>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>