rtc: rv3032: allow setting BSM

Backup Switch Mode is currently set properly when the trickle charger is
enabled. However, in the case of a non-rechargeable battery, it is
necessary to be able to enable it, only allow that when the trickle charger
is disabled.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211018151933.76865-8-alexandre.belloni@bootlin.com