pico: reset offset in SAVE command init

Really useful for sending more than one file...

32blit-pico/usb.cpp

diff --git a/32blit-pico/usb.cpp b/32blit-pico/usb.cpp
index 5e00734..47ea1d7 100644
--- a/32blit-pico/usb.cpp
+++ b/32blit-pico/usb.cpp
@@ -176,6 +176,7 @@
 
   void init() override {
     parse_state = ParseState::Filename;
+    file_offset = 0;
     buf.reset();
   }