power: qpnp-linear-charger: fix USB detection logic

Charger driver uses 'USB_CHG_PTH_STS' register to detect
USB insertion/removal status.
Update the USB detection logic to use 'INT_RT_STS' register
which has better de-bounced 'usbin_valid' signal as compared
to 'USB_CHG_PTH_STS' register.

CRs-Fixed: 709183
Change-Id: I9903b8708cfaa7a04c96aa984624eb0ca867adbf
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>