Added default transactional spi impl, patch for demonstration

Author: ryankurte

Cargo.toml

@@ -123,3 +123,6 @@ required-features = ["rt", "stm32f429"]
 [[example]]
 name = "rng-display"
 required-features = ["rt", "stm32f407"]
+
+[patch.crates-io]
+embedded-hal = { git = "https://github.com/ryankurte/embedded-hal.git", branch = "feature/spi-transactions" }

src/spi.rs

@@ -1059,3 +1059,8 @@ impl<SPI, PINS> embedded_hal::blocking::spi::write::Default<u8> for Spi<SPI, PIN
     SPI: Deref<Target = spi1::RegisterBlock>
 {
 }
+
+impl<SPI, PINS> embedded_hal::blocking::spi::transactional::Default<u8> for Spi<SPI, PINS> where
+    SPI: Deref<Target = spi1::RegisterBlock>
+{
+}