spi/fsl-espi: disable CONFIG_SPI_FSL_ESPI=m build

When spi_fsl_espi is chosen to be built as a module, there is a build
error because we test only CONFIG_SPI_FSL_ESPI in declaration of
struct mpc8xxx_spi in drivers/spi/spi_fsl_lib.h. Also some called
functions are not exported.

So we forbid CONFIG_SPI_FSL_ESPI to be tristate here.

The error looks like:
drivers/spi/spi_fsl_espi.c: In function 'fsl_espi_bufs':
drivers/spi/spi_fsl_espi.c:232: error: 'struct mpc8xxx_spi' has no member named 'len'
...

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>