hwmon: (lm90) Drop redundant safety on cache lifetime

hwmon: (lm90) Drop redundant safety on cache lifetime

time_after (as opposed to time_after_equal) already ensures that the
cache lifetime is at least as much as requested. There is no point in
manually adding another jiffy to that value, and this can confuse the
reader into wrong interpretation.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Imre Deak <imre.deak@intel.com>