media: cec.h: increase input_phys buffer

Fixes this compiler warning:

drivers/media/cec/core/cec-core.c: In function 'cec_allocate_adapter':
drivers/media/cec/core/cec-core.c:317:21: warning: '/input0' directive output may be truncated writing 7 bytes into a region of size between 1 and 32 [-Wformat-truncation=]
  317 |                  "%s/input0", adap->name);
      |                     ^~~~~~~
drivers/media/cec/core/cec-core.c:316:9: note: 'snprintf' output between 8 and 39 bytes into a destination of size 32
  316 |         snprintf(adap->input_phys, sizeof(adap->input_phys),
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  317 |                  "%s/input0", adap->name);
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>