hw/ssi: ibex_spi: fixup coverity issue

hw/ssi: ibex_spi: fixup coverity issue

This patch addresses the coverity issues specified in [1],
as suggested, `FIELD_DP32()`/`FIELD_EX32()` macros have been
implemented to clean up the code.

[1] https://www.mail-archive.com/qemu-devel@nongnu.org/msg887713.html

Fixes: Coverity CID 1488107
Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-Id: <20220930033241.206581-2-wilfred.mallawa@opensource.wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>