media: em28xx/em28xx.h: Remove duplicate struct declaration

struct em28xx is declared at 219th line.
The declaration here is unnecessary. Remove it.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>