es51919 lcr: unbreak channel setup after successful detection

Commit 6bcb3ee8763bb introduced initial support for the Cyrustek ES51919
chipset. Its setup_channels() routine used to init a variable to assume
failure, then a loop added channels and changed the value to success.

Commit 5e23fcab889c6 changed channel setup to never fail, but kept the
initialization with an error code. Which prevented the operation of
successfully detected LCR meters.

Remove the no longer needed variable, instead always return success from
an operation which cannot fail.

Fixes: 5e23fcab889c6 "Simplify channel creation."
Signed-off-by: Gerhard Sittig <redacted>