spi/ssp: gate blocking SPI write behind feature

Cargo.toml

@@ -42,6 +42,7 @@ version = "0.5.1"
 default = ["stable"]
 stable = []
 nightly = []
+ssp-write-only = []
 
 [package.metadata.docs.rs]
 features = ["timer-queue"]

src/spi/ssp.rs

@@ -137,7 +137,8 @@ macro_rules! hal_ssp {
 
             // This will fill up and overrun the read FIFO, but it could be useful for use cases
             // where the master _never_ wants to read.
-            //impl embedded_hal::blocking::spi::write::Default<u8> for SpiSsp<$SSP> {}
+            #[cfg(feature = "ssp-write-only")]
+            impl embedded_hal::blocking::spi::write::Default<u8> for SpiSsp<$SSP> {}
         )+
     }
 }