gpio: aspeed: Add the flush write to ensure the write complete.

gpio: aspeed: Add the flush write to ensure the write complete.

Performing a dummy read ensures that the register write operation is fully
completed, mitigating any potential bus delays that could otherwise impact
the frequency of bitbang usage. E.g., if the JTAG application uses GPIO to
control the JTAG pins (TCK, TMS, TDI, TDO, and TRST), and the application
sets the TCK clock to 1 MHz, the GPIO's high/low transitions will rely on
a delay function to ensure the clock frequency does not exceed 1 MHz.
However, this can lead to rapid toggling of the GPIO because the write
operation is POSTed and does not wait for a bus acknowledgment.

Fixes: 361b79119a4b ("gpio: Add Aspeed driver")
Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20241008081450.1490955-2-billy_tsai@aspeedtech.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>