staging: ath6kl: Fixing a compile error

The commit fixes a compilation error that was encountered while using
a specific kernel configuration file. The problem was the use of some
functions defined in <linux/semaphore.h> without including the header
file explicitly. It was probably working before because of the
dependency getting implicitly satisfied via some other header file.

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Vipin Mehta <vmehta@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>