msf2: Remove dead code reported by Coverity

Fixed incorrect frame size mask, validated maximum frame
size in spi_write and removed dead code.

Signed-off-by: Subbaraya Sundeep <sundeep.lkml@gmail.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1508898544-10307-1-git-send-email-sundeep.lkml@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
