[media] tw68: simplify tw68_buffer_count

The code to calculate the maximum number of buffers allowed in 4 MB
is 1) wrong if PAGE_SIZE != 4096 and 2) unnecessarily complex.

Fix and simplify the code.

Reported-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>