hw/ssi/imx_spi: Removed unnecessary cast of rx data received from slave

hw/ssi/imx_spi: Removed unnecessary cast of rx data received from slave

When inserting the value retrieved (rx) from the spi slave, rx is pushed to
rx_fifo after being cast to uint8_t. rx_fifo is a fifo32, and the rx
register the driver uses is also 32 bit. This zeroes the 24 most
significant bits of rx. This proved problematic with devices that expect to
use the whole 32 bits of the rx register.

Signed-off-by: Eden Mikitas <e.mikitas@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>