USB: Fix bug in ext_chg_active flag handling

The ext_chg_active flag is set to true when the external charger
processing is pending. It is set to true when DCP is detected and
set to false when the user space notifies that it is done with
the charger detection. The user space may unblock the low power mode
any time to access the hardware to request a new voltage setting.
The current code does not set ext_chg_active flag in this case due to
which USB enters low power mode while external charger processing
is pending. This results in unclocked register access.
Fix the issue by setting ext_chg_active to true every time the
userspace calls the block LPM.

CRs-Fixed: 663301
Change-Id: If18f762d7fc0b4eaba297e27bbd7382b161fbcde
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>