Finish converting GSL to absl::Span

One less dependency.

Change-Id: Iaac25a3c1312a21a8a2b77b8cfee2463bdb51196
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/motors/print/usb.h b/motors/print/usb.h
index 981c5d3..1ed1bee 100644
--- a/motors/print/usb.h
+++ b/motors/print/usb.h
@@ -17,11 +17,11 @@
 
   void Initialize() override;
 
-  int WriteStdout(gsl::span<const char> buffer) override {
+  int WriteStdout(absl::Span<const char> buffer) override {
     return stdout_tty_->Write(buffer.data(), buffer.size());
   }
 
-  int WriteDebug(gsl::span<const char> buffer) override {
+  int WriteDebug(absl::Span<const char> buffer) override {
     if (debug_tty_ == nullptr) {
       return buffer.size();
     }
@@ -48,11 +48,11 @@
 
   void Initialize() override;
 
-  int WriteStdout(gsl::span<const char> buffer) override {
+  int WriteStdout(absl::Span<const char> buffer) override {
     return stdout_tty_.Write(buffer.data(), buffer.size());
   }
 
-  int WriteDebug(gsl::span<const char> buffer) override {
+  int WriteDebug(absl::Span<const char> buffer) override {
     return debug_tty_.Write(buffer.data(), buffer.size());
   }