V4L: saa7146: Fix allocation of clipping memory

V4L: 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.

(cherry picked from commit 7a7cd1920969dd9da4e0d99aab573b3eba24c799)

Thanks-to: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>