power: smb138x-charger: set buck freq to 700kHz in slave mode

Currently the buck frequency is set to 700kHz only when the smb138x is
in master mode. The buck frequency needs to be 700kHz in both master and
slave mode to prevent ADC errors. Do it.

Change-Id: Ib2cea9b38b6b23f9f229ee85c2bde5a5fd6220fd
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>