The register access to SMPR1/SMPR2 was incorrect in three aspects:
1. For channels < 10, SMPR1 was cleared but SMPR2 should have been
cleared
2. The code was not thread-safe
3. An unneeded write was issued. (The compiler won't combine the
in-place bitwise operations into a single read-modify-write
sequence on `volatile` memory.)
Fixes https://github.com/RIOT-OS/RIOT/issues/20261
4ed287cec8