spi: re-export spi MODEs defined in embedded_hal.

SPI mode is defined by CPOL and CPHA parameters, where CPOL determines
the polarity of the clock and CPHA determines the timing of the data
bits relative to the clock pulses.

Constants for all four SPI modes are defined in embedded_hal. Re-export
them from bitbang_hal to simplify its usage: now there is no no need
to pull embedded_hal as dependency just to use MODE_* consts.

Besides, update spi example accordingly.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>

examples/spi.rs

@@ -5,14 +5,15 @@
 use panic_halt;
 
 use metro_m4 as hal;
-use embedded_hal;
 use hal::clock::GenericClockController;
 use hal::delay::Delay;
 use hal::prelude::*;
 use hal::{entry, CorePeripherals, Peripherals};
 use hal::timer::TimerCounter;
 use nb::block;
-use bitbang_hal;
+
+use bitbang_hal::spi::SPI;
+use bitbang_hal::spi::MODE_0;
 
 #[entry]
 fn main() -> ! {
@@ -39,12 +40,7 @@ fn main() -> ! {
     let mosi = pins.mosi.into_push_pull_output(&mut pins.port);
     let sck = pins.sck.into_push_pull_output(&mut pins.port);
 
-    let mode = embedded_hal::spi::Mode {
-        polarity: embedded_hal::spi::Polarity::IdleLow,
-        phase: embedded_hal::spi::Phase::CaptureOnFirstTransition,
-    };
-    let mut spi = bitbang_hal::spi::SPI::new(mode, miso, mosi, sck, timer);
-
+    let mut spi = SPI::new(MODE_0, miso, mosi, sck, timer);
     let mut delay = Delay::new(core.SYST, &mut clocks);
 
     loop {

src/spi.rs

@@ -1,3 +1,5 @@
+pub use embedded_hal::spi::{MODE_0, MODE_1, MODE_2, MODE_3};
+
 use embedded_hal::spi::{FullDuplex, Mode, Phase::*, Polarity::*};
 use embedded_hal::digital::{InputPin, OutputPin};
 use embedded_hal::timer::{CountDown, Periodic};