usb: common: move function's kerneldoc next to its definition

Following a general rule, add the kerneldoc for a function next
to it's definition, but not next to its declaration in a header
file.

Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/c4d2e010ae2bf67cdfa0b55e6d1deb9339d9d3dc.1615170625.git.chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>