gpio: acpi: work around false-positive -Wstring-overflow warning

gcc-7 notices that the pin_table is an array of 16-bit numbers,
but fails to take the following range check into account:

drivers/gpio/gpiolib-acpi.c: In function 'acpi_gpiochip_request_interrupt':
drivers/gpio/gpiolib-acpi.c:206:24: warning: '%02X' directive writing between 2 and 4 bytes into a region of size 3 [-Wformat-overflow=]
   sprintf(ev_name, "_%c%02X",
                        ^~~~
drivers/gpio/gpiolib-acpi.c:206:20: note: directive argument in the range [0, 65535]
   sprintf(ev_name, "_%c%02X",
                    ^~~~~~~~~
drivers/gpio/gpiolib-acpi.c:206:3: note: 'sprintf' output between 5 and 7 bytes into a destination of size 5
   sprintf(ev_name, "_%c%02X",
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
    agpio->triggering == ACPI_EDGE_SENSITIVE ? 'E' : 'L',
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pin);
    ~~~~

As suggested by Andy, this changes the format string to have a fixed length.
Since modifying the range check did not help, I also opened a bug against
gcc, see link below.

Fixes: 0d1c28a4

 ("gpiolib-acpi: Add ACPI5 event model support to gpio.")
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patchwork.kernel.org/patch/9840801/
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82123
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>