mt76: mt76x0e: make array mt76x0_chan_map static const, makes object smaller

Don't populate the array mt76x0_chan_map on the stack but instead make it
static const. Makes the object code smaller by 80 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
   7685	   1192	      0	   8877	   22ad	mediatek/mt76/mt76x0/eeprom.o

After:
   text	   data	    bss	    dec	    hex	filename
   7541	   1256	      0	   8797	   225d	mediatek/mt76/mt76x0/eeprom.o

(gcc version 9.2.1, amd64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>