ltq-vdsl-vr9-mei: fix ignored-qualifiers build warnings

ltq-vdsl-vr9-mei: fix ignored-qualifiers build warnings

These functions return the register memory addresses. Usually
they are fixed values, hence I think it's safe to remove the
qualifier "volatile".

Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/18744
Signed-off-by: Robert Marko <[email protected]>
(cherry picked from commit bcf3f96c024872d2ab67b5f0e36f650e0993d1d8)
Signed-off-by: Nora Matthias Schiffer <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20440
Signed-off-by: Hauke Mehrtens <[email protected]>