drivers: spi:ti_qspi: change ti_qspi_slave to ti_qspi_priv for driver model conversion

Changing the ti_qspi_priv structure and its instance names from
to priv for driver mode conversion.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>