MdeModulePkg/UsbBusDxe: log warning message at DEBUG_WARN level

MdeModulePkg/UsbBusDxe: log warning message at DEBUG_WARN level

"UsbSelectConfig: failed to connect driver %r, ignored" is an error
message, but it states at once that the error condition will not affect
the control flow. Degrade the report to DEBUG_WARN.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>