media: msi2500: assign SPI bus number dynamically

SPI bus number must be assigned dynamically for each device, otherwise it
will crash when multiple devices are plugged to system.

Reported-and-tested-by:  <syzbot+c60ddb60b685777d9d59@syzkaller.appspotmail.com>

Cc: stable@vger.kernel.org
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>