mfd: intel_quark_i2c_gpio: Don't crash if !DMI

dmi_get_system_info() may return NULL either when CONFIG_DMI is not set or when
board has an old firmware. The patch prevents a crash and changes the default
frequency to be in align with older board.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[Lee: Removed overt "sentinel" comment and extra lines]
Signed-off-by: Lee Jones <lee.jones@linaro.org>