ath5k: reset key cache after resume

Otherwise it may be impossible to connected to an open network after a
resume.

This is a modified version of an original patch by
Alex Eskin <alexeskin@yahoo.com>:

	https://bugzilla.redhat.com/show_bug.cgi?id=425950#c8



Signed-off-by: John W. Linville <linville@tuxdriver.com>