usb: gadget: limit the input string size in fsg_show_file

Fix code quality issues found by the automatic checking tool.
Limit the input string size in fsg_show_file for safe usage of memmove

CRs-Fixed: 566927
Change-Id: I0feed4877c0d8075460e7dd75e01967a68ce9db3
Signed-off-by: Danny Segal <dsegal@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>