uart: fix enum references

Since Doxygen 1.9.5 @ref enum::FIELD references will not work. Use the
`#FIELD` notation instead. Since enums place all of its entries to the
global namespace, adding the enum only adds redundant information
Doxygen can figure out on its own.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>