usb: gadget: move utf8_to_utf16le to header file

As other users may use utf8_to_utf16le() to convert the utf8
to utf16 for usb, so move it to head file.

Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>