iio: adc: adc12138: make array ch_to_mux static, makes object code smaller

Don't populate const array ch_to_mux on the stack, instead
make it static. Makes the object code smaller by over 200 bytes:

Before:
text data bss dec hex filename
12663 1648 128 14439 3867 drivers/iio/adc/ti-adc12138.o

After
text data bss dec hex filename
12353 1744 128 14225 3791 drivers/iio/adc/ti-adc12138.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ti-adc12138.c

···
 				 void *data, int len)
 
 {
-	const u8 ch_to_mux[] = { 0, 4, 1, 5, 2, 6, 3, 7 };
+	static const u8 ch_to_mux[] = { 0, 4, 1, 5, 2, 6, 3, 7 };
 	u8 mode = (ch_to_mux[channel->channel] << 4) |
 		  (channel->differential ? 0 : 0x80);