media: stk1160: fix number of buffers in case not all buffers are created

In case we fail to allocate a transfer_buffer then we
break the buffers creation loop and update the number of
buffers to the number of successfully allocated which should
be 'i' and not 'i - 1' nor 'i + 1'

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>