docs: usb: replace some characters

The conversion tools used during DocBook/LaTeX/html/Markdown->ReST
conversion and some cut-and-pasted text contain some characters that
aren't easily reachable on standard keyboards and/or could cause
troubles when parsed by the documentation build system.

Replace the occurences of the following characters:

	- U+feff (''): BOM
	  as it is not needed on UTF-8
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/0a4b0c38a9cd1133402a04a7ff60fefd9682d42e.1623826294.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/usb/ehci.rst

-===========
+===========
 EHCI driver
 ===========
 

Documentation/usb/gadget_printer.rst

-===============================
+===============================
 Linux USB Printer Gadget Driver
 ===============================