atmega_common/gpio_init: fix pull-up settings for GPIO_IN_PU

This Patch makes gpio_init precalculate the pin mask once,
This Patch makes gpio_init of atmega_common configure the pin as an input and configure the pullup in the case of GPIO_IN_PU.
GPIO_IN_PU and GPIO_IN need to change pullup so they need to change output (this is coverd by the not touching outputs is not guaranteed statement)
(this is a special case for atmega the pull_up is configured by writing 1 to the port_register which is also the level of the output if the pin is configured to output).

This fix makes it more compliant to comments in periph/gpio.h

cpu/atmega_common/periph/gpio.c

@@ -127,16 +127,18 @@ static inline int8_t _int_num(gpio_t pin)
 
 int gpio_init(gpio_t pin, gpio_mode_t mode)
 {
+    uint8_t pin_mask = (1 << _pin_num(pin));
     switch (mode) {
         case GPIO_OUT:
-            _SFR_MEM8(_ddr_addr(pin)) |= (1 << _pin_num(pin));
+            _SFR_MEM8(_ddr_addr(pin)) |= pin_mask;
             break;
         case GPIO_IN:
-            _SFR_MEM8(_ddr_addr(pin)) &= ~(1 << _pin_num(pin));
-            _SFR_MEM8(_port_addr(pin)) &= ~(1 << _pin_num(pin));
+            _SFR_MEM8(_ddr_addr(pin)) &= ~pin_mask;
+            _SFR_MEM8(_port_addr(pin)) &= ~pin_mask;
             break;
         case GPIO_IN_PU:
-            _SFR_MEM8(_port_addr(pin)) |= (1 << _pin_num(pin));
+            _SFR_MEM8(_ddr_addr(pin)) &= ~pin_mask;
+            _SFR_MEM8(_port_addr(pin)) |= pin_mask;
             break;
         default:
             return -1;