i read that TL-WN550G/TL-WN551G
Working fine with MadWiFi SVN 20080109 under Ubuntu server 7.10. Running in master mode without any problems.
but when tried to set it using under ubuntu 8.04 server
using madwifi driver
root@samo:~# iwconfig ath0 mode Master
Error for wireless request "Set Mode" (8B06) :
    SET failed on device ath0 ; Invalid argument.

when tested it using hostap
root@samo:~# hostapd /etc/hostapd/hostapd.conf
Configuration file: /etc/hostapd/hostapd.conf
ioctl[SIOCSIWMODE]: Invalid argument
Could not set interface to master mode!
madwifi driver initialization failed.

are you sure it is working ?