MAINTAINERS: Update entry for st7703 driver after the rename

The driver was renamed, change the path in the MAINTAINERS file.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://lore.kernel.org/lkml/20200701184640.1674969-1-megous@megous.com/#t