drivers: i2c_dw: fix request_bytes overflow when receiving > 256 bytes

The original definition of request_bytes as uint8_t caused incorrect
behavior when attempting to receive more than 256 bytes, as the variable
would overflow. This patch changes its type to uint32_t to allow
correct tracking of large I2C transfers.

Signed-off-by: Eason Huang <eason.huang@tronfuture.com>