V4L/DVB (5441): Saa7146: Fix allocation of clipping memory

Olaf Hering pointed out that SAA7146_CLIPPING_MEM would become
very large for PAGE_SIZE > 4K.
In fact, the number of clipping windows is limited to 16,
and calculate_clipping_registers_rect() does not use more
than 256 bytes. SAA7146_CLIPPING_MEM adjusted accordingly.

Thanks-to: Olaf Hering <olaf@aepfle.de>
Acked-by: Michael Hunold <hunold@linuxtv.org>
Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>