i2c: stu300: Use proper printk format for iomem pointer

iomem pointers should be printed with pointer format to hide the
actual value and fix warnings when compiling on 64-bit platform (e.g. with
COMPILE_TEST):

    drivers/i2c/busses/i2c-stu300.c: In function ‘stu300_wait_while_busy’:
    drivers/i2c/busses/i2c-stu300.c:446:76: warning:
        cast from pointer to integer of different size [-Wpointer-to-int-cast]

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>