spi: fsi: restore CONFIG_FSI dependency

spi: fsi: restore CONFIG_FSI dependency

The assumption that the build dependency was not necessary turned
out to be wrong, as building SPI_FSI without FSI results in a link
failure:

aarch64-linux-ld: drivers/spi/spi-fsi.o: in function `fsi_spi_check_status':
spi-fsi.c:(.text+0x54): undefined reference to `fsi_device_read'
aarch64-linux-ld: drivers/spi/spi-fsi.o: in function `fsi_spi_read_reg':
spi-fsi.c:(.text+0x120): undefined reference to `fsi_device_write'
aarch64-linux-ld: spi-fsi.c:(.text+0x170): undefined reference to `fsi_device_read'

Fixes: f916c7080d28 ("spi: fsi: Make available for build test")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230310140605.569363-1-arnd@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>