[media] tuners: Refactoring for m88rs6000t_sleep()

This issue was detected by using the Coccinelle software.

1. Let us return directly if a call of the regmap_write() function failed.

2. Delete the jump label "err" then.

3. Return zero as a constant at the end.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>