power: qpnp-charger: Avoid override USBIN_COMP in reverse boost for SMBBP

When SMBBP working at reverse boost mode with a charger connected, USBIN
comparator can be used to turn off the OVP FET and trigger an USBIN_VALID
IRQ to detect the charger removal.

Avoid overriding USBIN comparator when SMBBP working at reverse boost
mode, to ensure the charger insertion and removal can be detected.

Change-Id: If705bc5cb7132ab2ce9b97ac4b887a4cdc201b52
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>