hw/ssi: imx_spi: Rework imx_spi_read() to handle block disabled

When the block is disabled, it stay it is 'internal reset logic'
(internal clocks are gated off). Reading any register returns
its reset value. Only update this value if the device is enabled.

Ref: i.MX 6DQ Applications Processor Reference Manual (IMX6DQRM),
     chapter 21.7.3: Control Register (ECSPIx_CONREG)

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Message-id: 20210129132323.30946-6-bmeng.cn@gmail.com
Message-Id: <20210115153049.3353008-5-f4bug@amsat.org>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>