usb: pd: include kernel.h

This makes life a bit easier for the drivers that include
pd.h. All pd_header_*_le() inline functions defined in pd.h
call le16_to_cpu(), and all *_LE() macros in pd.h call
cpu_to_le16().

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>