V4L/DVB (5392): Zr364xx: Use kernel's byte-swapping function

Some code to swap bytes wasn't using the swab16() function that the
kernel provides for this.  Make use of it, which results in more
efficient code.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Acked-by: Antoine Jacquet <royale@zerezo.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>