usb: host: imx21-hcd: Demote function header which is clearly not kerneldoc

usb: host: imx21-hcd: Demote function header which is clearly not kerneldoc

No attempt has been made to document the demoted function here.

Fixes the following W=1 kernel build warning(s):

 drivers/usb/host/imx21-hcd.c:233: warning: Function parameter or member 'imx21' not described in 'copy_to_dmem'
 drivers/usb/host/imx21-hcd.c:233: warning: Function parameter or member 'dmem_offset' not described in 'copy_to_dmem'
 drivers/usb/host/imx21-hcd.c:233: warning: Function parameter or member 'src' not described in 'copy_to_dmem'
 drivers/usb/host/imx21-hcd.c:233: warning: Function parameter or member 'count' not described in 'copy_to_dmem'

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: Stephen Boyd <swboyd@chromium.org>
Cc: Jay Monkman <jtm@lopingdog.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200706133341.476881-17-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>