Staging: comedi: comedi_buf: Replace 'unsigned' with 'unsigned int'

Fix checkpatch warning:
Prefer 'unsigned int' to bare use of 'unsigned'
in file comedi_buf.c

Signed-off-by: Leslie Klein <lesliebklein@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>