meta-quanta: fix shellcheck issues

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I189baf142cc86d44ebbf615edd3bb0ec1785be8d
diff --git a/meta-quanta/meta-q71l/recipes-phosphor/quanta-powerctrl/files/poweroff.sh b/meta-quanta/meta-q71l/recipes-phosphor/quanta-powerctrl/files/poweroff.sh
index 8e0dae3..5049f41 100755
--- a/meta-quanta/meta-q71l/recipes-phosphor/quanta-powerctrl/files/poweroff.sh
+++ b/meta-quanta/meta-q71l/recipes-phosphor/quanta-powerctrl/files/poweroff.sh
@@ -1,7 +1,7 @@
 #!/bin/bash
 
 GPIO_BASE=$(cat /sys/devices/platform/ahb/ahb:apb/1e780000.gpio/gpio/*/base)
-GPIO_NUM=$(($GPIO_BASE + 24 + 3))
+GPIO_NUM=$((GPIO_BASE + 24 + 3))
 
 echo 1 > /sys/class/gpio/gpio${GPIO_NUM}/value
 sleep 5