[media] xc4000: remove XREG_BUSY code only supported in xc5000

The xc4000 driver is based on the original xc5000 driver, and while the
xc5000 supports the XREG_BUSY register, the xc4000 does not.  So remove the
code in question.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>