tuna: set minimum brightness to 1 instead of 10

Minimum brightness for toroplus (and other tuna-based devices) was
reported as being too bright for night-time usage. It makes sense
logically that the minimum brightness that can be set should be 1
instead of 10, this change is also present in mako.

Change-Id: Id818cdb3a4151e647409f31a3529bf5dbadbed02
JIRA: CYAN-2319