usb: gadget: u_ether: use <linux/hex.h> header file

usb: gadget: u_ether: use <linux/hex.h> header file

Since <linux/kernel.h> no longer includes <linux/hex.h>, any users
of the hex interfaces need to include <linux/hex.h> themselves, so
add the header file here as needed.

Suggested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: e065c6a7e46c ("usb: gadget: u_ether: add gether_opts for config caching")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/2768c835-7ac9-4540-a665-5dd516a80eee@infradead.org/T/#t
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20260113071831.4158296-1-rdunlap@infradead.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>