feature i2c fails to compile if feature spi isn't enabed

[webhet]

Hi,
very cool and well designed crate.

Just noticed a problem when wanting to use the i2c feature only. This doesn't compile and gives the error:

error[E0432]: unresolved import `spi`
   --> /home/.../.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/bma400-0.1.0/src/lib.rs:193:9
    |
193 | pub use spi::SPIInterface;
    |         ^^^ help: a similar path exists: `embedded_hal::spi`

I think L193 should use cfg(...spi...) instead of i2c.

[cfrenette]

You are correct @webhet, good catch and thank you for reporting this! I've made the changes above. I'll merge them into a new release and publish to crates.io shortly.