mfd: cros-ec: copy the whole event in get_next_event_xfer

mfd: cros-ec: copy the whole event in get_next_event_xfer

Commit ebbdcd9f47363d08f49c679e1eddb136683fd290 caused cros-ec keyboard events
be truncated on many chromebooks so that Left and Right keys on Column 12 were
always 0. Use ret as memcpy len to fix this.

The old code was using ec_dev->event_size, which is the event payload/data size
excluding event_type header, for the length of the memcpy operation. Use ret
as memcpy length to avoid the off by one and copy the whole msg->data.

Fixes: ebbdcd9f4736 ("mfd: cros-ec: Increase maximum mkbp event size")
Acked-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Emil Renner Berthing <kernel@esmil.dk>
Signed-off-by: Emil Karlson <jekarlson@gmail.com>
Signed-off-by: Benson Leung <bleung@chromium.org>