rtl8xxxu: make arrays static, makes object smaller

Don't populate const arrays on the stack but instead make them
static. Makes the object code smaller by 60 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  15133	   8768	      0	  23901	   5d5d	realtek/rtl8xxxu/rtl8xxxu_8192e.o
  15209	   6392	      0	  21601	   5461	realtek/rtl8xxxu/rtl8xxxu_8723b.o
 103254	  31202	    576	 135032	  20f78	realtek/rtl8xxxu/rtl8xxxu_core.o

After:
   text	   data	    bss	    dec	    hex	filename
  14861	   9024	      0	  23885	   5d4d	realtek/rtl8xxxu/rtl8xxxu_8192e.o
  14953	   6616	      0	  21569	   5441	realtek/rtl8xxxu/rtl8xxxu_8723b.o
 102986	  31458	    576	 135020	  20f6c	realtek/rtl8xxxu/rtl8xxxu_core.o

(gcc version 9.2.1, amd64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>