spi_mpc8xxx: Turn qe_mode into flags

Soon there will be more flags introduced in subsequent patches, so
let's turn qe_mode into flags.

Also introduce mpc8xxx_spi_strmode() and print current SPI mode.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>