drivers: display: st7735r: fix build break

display_st7735r broke after cd1fb297:

/zephyr/drivers/display/display_st7735r.c: In function 'st7735r_write':
/zephyr/drivers/display/display_st7735r.c:191:34: error: initialization
discards 'const' qualifier from pointer target type
[-Werror=discarded-qualifiers]
  191 |  struct st7735r_config *config = dev->config;
      |                                  ^~~
/zephyr/drivers/display/display_st7735r.c: In function 'st7735r_lcd_init':
/zephyr/drivers/display/display_st7735r.c:318:26: error: passing
argument 1 of 'st7735r_set_lcd_margins' from incompatible pointer type
[-Werror=incompatible-po
inter-types]
  318 |  st7735r_set_lcd_margins(dev, data->x_offset, data->y_offset);
      |                          ^~~
      |                          |
      |                          const struct device *
/zephyr/drivers/display/display_st7735r.c:60:58: note: expected 'struct
st7735r_data *' but argument is of type 'const struct device *'
   60 | static void st7735r_set_lcd_margins(struct st7735r_data *data,
      |                                     ~~~~~~~~~~~~~~~~~~~~~^~~~
/zephyr/drivers/display/display_st7735r.c: In function 'st7735r_init':
/zephyr/drivers/display/display_st7735r.c:441:24: error: passing
argument 1 of 'device_is_ready' from incompatible pointer type
[-Werror=incompatible-pointer-types]
  441 |   if (!device_is_ready(&config->reset)) {
      |                        ^~~~~~~~~~~~~~
      |                        |
      |                        const struct gpio_dt_spec *

and few others. Fix all of them.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>