xhci: replace xhci_info() with xhci_dbg()

This patch replaces the calls to xhci_info() with calls to
xhci_dbg() and removes the unused xhci_info() definition
from xhci-hcd.

By replacing the xhci_info() with xhci_dbg(), the calls to
dev_info() are replaced with calls to dev_dbg() so that
their output can be dynamically controlled via the dynamic
debugging mechanism.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>