[media] xc5000: Remove the global mutex lock at xc5000 firmware init

As reported by Devin Heitmueller <dheitmueller@kernellabs.com>:

> It seems like a change such as this could significantly change the
> timing of tuner initialization if you have multiple xc5000 based
> products that might have a slow i2c bus.  Was that intentional?

After discussed with Eddi de Pierri <eddi@depieri.net>, it was pointed that
the change was not intentional, and it was just a trial while developing
the patches that add support for HVR-930C.

So, remove this hack.

Reported-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Acked by: Eddi de Pierri <eddi@depieri.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>