kernel: fix kernel >4.4 compatibility

Compiling on kernel >=4.5 produces the following error:

  CC [M]  spec-sw/kernel/wr-nic-gpio.o
spec-sw/kernel/wr-nic-gpio.c: In function ‘gc_to_fmc’:
spec-sw/kernel/wr-nic-gpio.c:20:25: error: ‘struct gpio_chip’ has no member named ‘dev’
  struct device *dev = gc->dev;
                         ^
spec-sw/kernel/wr-nic-gpio.c: In function ‘wrn_gpio_init’:
spec-sw/kernel/wr-nic-gpio.c:75:4: error: ‘struct gpio_chip’ has no member named ‘dev’
  gc->dev = &fmc->dev;
    ^

This is due to a change in struct gpio_chip introduced in 4.5,
where the "dev" field was renamed to "parent".

58383c7 gpio: change member .dev to .parent

This patch makes our code compatible with modern kernel versions

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
Signed-off-by: Dimitris Lampridis <Dimitris.Lampridis@cern.ch>