usb: dwc3: Enable parkmode for Gen1 controllers

Commit 8799bc1e ("usb: dwc3: Disable parkmode for Gen1 controllers")
was added to disable the park mode which is a debug feature. But, doing
this indirectly affect the host mode throughput for targets using Gen1
controllers in HS mode.

Therefore enabling park mode only for HS to improve the throughput
numbers in host mode.

Change-Id: Iea9e44d0ed0f9e2f9bba6731cae29229d9b46bb1
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>