pinmux: Fix using wrong variable name

In the pinmux_dev driver for the Quark SE development board in
pinmux_dev_set() the variable 'mode' was used, but it should have been
'func'. This was causing a compilation error:

/home/vinicius/work/zephyr/boards/quark_se_devboard/pinmux.c:
In function 'pinmux_dev_set':
/home/vinicius/work/zephyr/boards/quark_se_devboard/pinmux.c:245:23:
error: 'mode' undeclared (first use in this function)
  uint32_t mode_mask = mode << (pin_no << 1);

Change-Id: I5b9df7c6b488dc5b8819fcf59bb3b994d9d4820b
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>