gpio: intel_gpio: Fix register/bit offsets intel_gpio_get_value()

Fix the following in intel_gpio_get_value():

 * The value of the register is contained in the variable 'reg', not in
   'mode'. The variable 'mode' contains only the configuration whether
   the gpio is currently an input or an output.

 * The correct bitmasks for the input and output value are
   PAD_CFG0_RX_STATE and PAD_CFG0_TX_STATE.
   Use them instead of the currently used PAD_CFG0_RX_STATE_BIT and
   PAD_CFG0_TX_STATE_BIT.

Signed-off-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>