ath9k: correct sparse identified endian bug in ath_paprd_calibrate

drivers/net/wireless/ath/ath9k/main.c:282:26: warning: incorrect type in assignment (different base types)
drivers/net/wireless/ath/ath9k/main.c:282:26:    expected restricted __le16 [usertype] duration_id
drivers/net/wireless/ath/ath9k/main.c:282:26:    got int

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