hw/ssi/imx_spi: changed while statement to prevent underflow

The while statement in question only checked if tx_burst is not 0.
tx_burst is a signed int, which is assigned the value put by the
guest driver in ECSPI_CONREG. The burst length can be anywhere
between 1 and 4096, and since tx_burst is always decremented by 8
it could possibly underflow, causing an infinite loop.

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>