up_n_down - unused define removed and typo fix

I suspect MAX_MODE was used in the past to prevent the mode counter from
going over and allowing mode changes from any other mode, but currently
any number of clicks while not in MODE_OFF (or locked and new mode is
MODE_LOCK) will turn the light off anyway.  Re-introduce if mode changes
while already in a mode are enabled?
Also fixed a typo.
saving a few bytes - variable and ternary ops.

Re-using the current_level variable for end_light_level determination
saved 4 bytes.  Changing the if-then-else to ternary operators saved 6
more.  Whitespace for readability.