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imx8/gpio: allow 0 as "no-op" gpio pin

as the gpio controller number starts at 1,
we can use 0 to mean "no pin", so passing
0 to gpioout() or gpioin() pin argument
will have no effect.

--- a/sys/src/9/imx8/fns.h

+++ b/sys/src/9/imx8/fns.h

@@ -156,6 +156,6 @@

 extern uint iomuxgpr(int gpr, uint set, uint mask);

 /* gpio */

-#define GPIO_PIN(n, m)	(((n)-1)<<5 | (m))

+#define GPIO_PIN(n, m)	((n)<<5 | (m))

 extern void gpioout(uint pin, int set);

 extern int gpioin(uint pin);

--- a/sys/src/9/imx8/gpio.c

+++ b/sys/src/9/imx8/gpio.c

@@ -38,10 +38,13 @@

 static Ctlr*

 enable(uint pin)

 {

-	Ctlr *ctlr = &ctlrs[pin/32];

+	Ctlr *ctlr;

-	assert(ctlr < &ctlrs[nelem(ctlrs)]);

+	pin /= 32;

+	if(pin < 1 || pin > nelem(ctlrs))

+		return nil;

+	ctlr = &ctlrs[pin-1];

 	if(!ctlr->enabled){

 		setclkgate(ctlr->clk, 1);

 		ctlr->reg[GPIO_IMR] = 0;

@@ -48,7 +51,6 @@

 		ctlr->dir = ctlr->reg[GPIO_GDIR];

 		ctlr->enabled = 1;

 	}

-

 	return ctlr;

 }

@@ -57,7 +59,8 @@

 {

 	int bit = 1 << (pin % 32);

 	Ctlr *ctlr = enable(pin);

-

+	if(ctlr == nil)

+		return;

 	if((ctlr->dir & bit) == 0)

 		ctlr->reg[GPIO_GDIR] = ctlr->dir |= bit;

 	if(set)

@@ -71,7 +74,8 @@

 {

 	int bit = 1 << (pin % 32);

 	Ctlr *ctlr = enable(pin);

-

+	if(ctlr == nil)

+		return -1;

 	if(ctlr->dir & bit)

 		ctlr->reg[GPIO_GDIR] = ctlr->dir &= ~bit;

 	return (ctlr->reg[GPIO_DR] & bit) != 0;