drivers: timer: sam0_rtc_timer: Fix incorrect DT_INST conversion

clock-generator is a normal property.  To access it we should use
DT_INST_PROP(0, clock_generator) and not DT_INST_CLOCKS_CELL().

Fixes: #24399

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
1 file changed