gpio: gpio-ml-ioh: Fix missing ':' in 'struct ioh_gpio_reg_data

'struct ioh_gpio_reg_data's 'ien_reg' property is missing a ':'
which confuses the kerneldoc tooling/parsers/validators.

Replacing it squashes the following W=1 warning:

 drivers/gpio/gpio-ml-ioh.c:63: warning: Function parameter or member 'ien_reg' not described in 'ioh_gpio_reg_data'

Cc: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>