spi: ti_qspi: Fix SPI_3WIRE checking using mode

SPI_3WIRE is spi mode not spi flags, so this patch fixed
the spi-3wire checking throgh mode instead of flags.

Cc: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jagan Teki <jteki@openedev.com>