ath9k_htc: Use appropriate rs_datalen type

kernel test robot says:
drivers/net/wireless/ath/ath9k/htc_drv_txrx.c:987:20: sparse: warning: incorrect type in assignment (different base types)
drivers/net/wireless/ath/ath9k/htc_drv_txrx.c:987:20: sparse:    expected restricted __be16 [usertype] rs_datalen
drivers/net/wireless/ath/ath9k/htc_drv_txrx.c:987:20: sparse:    got unsigned short [usertype]
drivers/net/wireless/ath/ath9k/htc_drv_txrx.c:988:13: sparse: warning: restricted __be16 degrades to integer
drivers/net/wireless/ath/ath9k/htc_drv_txrx.c:1001:13: sparse: warning: restricted __be16 degrades to integer

Indeed rs_datalen has host byte order, so modify it's own type.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: cd486e62

 ("ath9k_htc: Discard undersized packets")
Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200808233258.4596-1-masashi.honma@gmail.com