cbus-tahvo: Add tahvo_write_reg locking

tahvo_write_reg() needs to take the mutex to avoid a race
condition with tahvo_set_clear_reg_bits:

tahvo_set_clear_reg_bits():   |  tahvo_write_reg():
    __tahvo_read_reg()        |
                              |      __tahvo_write_reg() <-- race here
    __tahvo_write_reg()       |

Signed-off-by: Michael Buesch <m@bues.ch>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>