gpio: aspeed: Fix incorrect offset of read back register.

The offset of the current read back register is the value of the gpio pin,
not the value written for the gpio output.
This patch fix it to avoid the other gpio output value controlled by the
same register being set incorrectly.

Fixes: 7ad889b0 ("gpio: Add Aspeed GPIO driver")
Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>