rtw89: rtw89_ser: add const to struct state_ent and event_ent

Change the struct and the uses to const to reduce data.

$ size drivers/net/wireless/realtek/rtw89/ser.o* (x86-64 defconfig w/ rtw89)
   text	   data	    bss	    dec	    hex	filename
   3741	      8	      0	   3749	    ea5	drivers/net/wireless/realtek/rtw89/ser.o.new
   3437	    312	      0	   3749	    ea5	drivers/net/wireless/realtek/rtw89/ser.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/2fd88e6119f62b968477ef9781abb1832d399fd6.camel@perches.com