Fix duplicate buffer in I2C code

[jamwaffles]

send_data() should be using less memory. @scowcron's implementation uses buffered chunks which might be a good way to go. Adding the initial 0x40 to the beginning of the buffer makes using another buffer a necessity at the moment.

[scowcron]

relevant, I think: rust-embedded/embedded-hal#47

[jamwaffles]

That looks good. Until that gets merged/implemented though, I think the best way to go is to port over your chunked tx buffer idea. 17 bytes isn't much, even for a uC.