spi: xilinx_spi: Fix to configure CPOL, CPHA mask

priv->mode is initialized when .set_speed triggers
with mode value, so checking mode for configuring
CPOL, CPHA using priv->mode is invalid hence use
mode from .set_speed argument, and at the end
priv->mode will initialized with mode.

This patch also replaces formatting string to use
speed instead of mode in .set_speed ops.

Signed-off-by: Jagan Teki <jteki@openedev.com>