qcom-charger: set optimal boost switching frequency

Currently, the default boost switching frequency is fixed at 800KHz,
which shows low efficiency in low USB load.

To improve it, only use the default 800KHz when boost current goes
above 100mA; otherwise, switch to 2MHz.

CRs-Fixed: 1095917
Change-Id: Iebb62303febfb78738ada4553fdb9b9eb4810ed5
Signed-off-by: Harry Yang <harryy@codeaurora.org>