Fixed an error in the previous commit

Author: CDarius

ports/atmel-samd/mpconfigport.mk

@@ -33,7 +33,7 @@ CIRCUITPY_BUILTINS_POW3 ?= 0
 CIRCUITPY_BUSDEVICE ?= 0
 CIRCUITPY_COMPUTED_GOTO_SAVE_SPACE ?= 1
 CIRCUITPY_COUNTIO ?= 0
-CIRCUITPY_KEYPAD_DEMUXKEYMATRIX = 0
+CIRCUITPY_KEYPAD_DEMUXKEYMATRIX ?= 0
 # Not enough RAM for framebuffers
 CIRCUITPY_FRAMEBUFFERIO ?= 0
 CIRCUITPY_FREQUENCYIO ?= 0