fastboot: change m_Maxsize_pre_cmd to 16KB in FBCopy

The Chipidea USB controller doesn't support transfer length bigger
than 16KB. So change m_Maxsize_pre_cmd to 16KB in FBCopy. This change
will not have performance drop compared to 64KB for High-speed and
Super-speed mode.

Signed-off-by: Xu Yang <[email protected]>

libuuu/fastboot.h

@@ -288,7 +288,7 @@ class FBCopy : public CmdBase
 private:
 	bool m_bDownload;
 	std::string m_local_file;
-	size_t m_Maxsize_pre_cmd = 0x10000;
+	size_t m_Maxsize_pre_cmd = 0x4000;
 	std::string m_target_file;
 };