Memory for "ah" field of "sc" structure is allocated via kzalloc() in ath5k_init_softc(). This function is called when driver is loaded and some wireless point is found (automatically) by the device. But it seems from the code that this memory is never freed after - there is no corresponded kfree for this field except error-path of ath5k_init_softc() itself.
Dynamic memory leaks detector from KEDR (http://kedr.berlios.de/) reports about leaks from that allocation in case of simple use case: loading driver, waiting several seconds, unloading driver.
Following freeing of block reported as leakage has not led to kernel failures - it seems that driver will not free allocated block neither directly nor indirectly. (This freeing was made with the kernel module designed by hand specially for that purpose.)
Here is the output of KEDR leak detector concerning this leaked memory block:
Block at 0xbe9dc000, size: 8192; stack trace of the allocation:
[<c0771d10>] ath5k_init_softc+0x132/0xf6f [ath5k]
[<c077667a>] cleanup_module+0x1e3/0x225 [ath5k]
A patch referencing this bug report has been merged in Linux v3.1-rc1:
Author: John W. Linville <email@example.com>
Date: Fri Jun 17 16:41:30 2011 -0400
ath5k: free sc->ah in ath5k_deinit_softc