iio: accel: fxls8962af: fix i2c dependency

iio: accel: fxls8962af: fix i2c dependency

With CONFIG_SPI=y and CONFIG_I2C=m, building fxls8962af into vmlinux
causes a link error against the I2C module:

aarch64-linux-ld: drivers/iio/accel/fxls8962af-core.o: in function `fxls8962af_fifo_flush':
fxls8962af-core.c:(.text+0x3a0): undefined reference to `i2c_verify_client'

Work around it by adding a Kconfig dependency that forces the SPI driver
to be a loadable module whenever I2C is a module.

Fixes: af959b7b96b8 ("iio: accel: fxls8962af: fix errata bug E3 - I2C burst reads")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210721151330.2176653-1-arnd@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>