어서 와, 어서 오세요, 어서 오십시오 are NOT(!) rude expressions at all
어서 just means "quickly" / "swiftly" / "promptly"
어서 와 => Literally means "Come quickly" (in a warm, welcoming sense - casual form) meaning "Welcome!"
어서 오세요 = 어서 오십시오 => Come quickly please (in a warm, welcoming sense - polite form) meaning "Welcome!"
어서 = hurry up
It's an expression that prompts a quick `빨리' (in terms of speed) or/and immediate '곧' (in terms of time) action.
경찰을 어서 불러요! = Hurry up and call the police!
This can be interpreted as
(1) Call the police as quickly as possible. Or
(2) Call the police as soon as possible. Or
The -요 verb ending determines the level of politeness, more or less similar to the English "Please".