drivers: serial: nrfx_uarte: Add missing closing bracket

A code ifdefed by UARTE_BAUDRATE_RETENTION_WORKAROUND was missing a
closing bracket. It was missed because CI did not compile any target
that is applying this workaround and enables code compilation.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>