rtc: palmas: Initialise bb_charging flag before using it

Initialise the variable high_bb_charging before using it to avoid
configuring wrong value and fix following compilation warning:

/*
rtc-palmas.c: In function ‘palmas_rtc_probe’:
rtc-palmas.c:242:7: warning: ‘high_bb_charging’ may be used
uninitialized in this function
*/

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>