spi: mpc8xxx_spi: fix missing dev_err definition

The build currently fails with

drivers/spi/mpc8xxx_spi.c:64:3: warning: implicit declaration of function ‘dev_err’ [-Wimplicit-function-declaration]
...
drivers/spi/built-in.o: In function `mpc8xxx_spi_set_speed':
drivers/spi/mpc8xxx_spi.c:227: undefined reference to `dev_err'

Fixes: 4856cc7a (mpc8xxx_spi: implement real ->set_speed)
Fixes: 1a7b462d (mpc8xxx_spi: put max_cs to use)
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>