Write data over SPI 1 byte at a time

Fixes #82

src/interface.rs

@@ -57,11 +57,15 @@ where
     ///
     /// Enables direct interaction with the device with the help of [command()](Epd4in2::command())
     pub(crate) fn data(&mut self, spi: &mut SPI, data: &[u8]) -> Result<(), SPI::Error> {
-        // high for data
-        let _ = self.dc.set_high();
+        for val in data.iter().copied() {
+            // high for data
+            let _ = self.dc.set_high();
 
-        // Transfer data (u8-array) over spi
-        self.write(spi, data)
+            // Transfer data one u8 at a time over spi
+            self.write(spi, &[val])?;
+        }
+
+        Ok(())
     }
 
     /// Basic function for sending [Commands](Command) and the data belonging to it.
@@ -86,11 +90,9 @@ where
         val: u8,
         repetitions: u32,
     ) -> Result<(), SPI::Error> {
-        // high for data
-        let _ = self.dc.set_high();
         // Transfer data (u8) over spi
         for _ in 0..repetitions {
-            self.write(spi, &[val])?;
+            self.data(spi, &[val])?;
         }
         Ok(())
     }