Ensure that drivers set max_num_buffers to a value >= 32.
For now there is no reason for drivers to request a lower
limit and doing so might potentially cause userspace issues.
Note that the old check of > MAX_BUFFER_INDEX was pointless
since q->max_num_buffers was already limited to MAX_BUFFER_INDEX
or less.

Also add a sanity check in __vb2_init_fileio(), returning
-ENOSPC if a driver returns more than 32 buffers from
VIDIOC_REQBUFS with count = q->min_reqbufs_allocation.

The vb2_fileio_data struct only supports up to 32 buffers,
so we need a check there.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>