staging: fbtft: replace master->setup() with spi_setup()

Calling the setup of the SPI master directly causes a NULL pointer
dereference with master drivers without a separate setup function.
This problem is reproduceable on ARM MXS platform.

So fix this issue by using spi_setup() instead.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>