set_dcsr_config(): Check which WARL bits could be set

(Originally discussed in https://github.com/riscv-collab/riscv-openocd/pull/1255#discussion_r2104538221)

The function `set_dcsr_config()` tries to set multiple WARL bits in the DCSR register to user-configured values.

Extend this function to read back the DCSR register. Warn the user if some of the WARL bits could not be set to the desired values.