drivers: spi_xmc4xxx: Add delay when changing clock polarity

The passive level of the clock does not change instanteneously when
it's set using function XMC_SPI_CH_ConfigureShiftClockOutput().
This means that the passive level of the clock can be in the wrong
state when the chip select goes low.

Fix this by adding a small delay when the polarity changes to allow
the clock to return to the proper level.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>