wifi: ath6kl: fix sparse warnings

Sparse warns:

drivers/net/wireless/ath/ath6kl/htc_pipe.c:241:17: warning: incorrect type in assignment (different base types)
drivers/net/wireless/ath/ath6kl/htc_pipe.c:241:17:    expected restricted __le16 x
drivers/net/wireless/ath/ath6kl/htc_pipe.c:241:17:    got unsigned short [usertype]
drivers/net/wireless/ath/ath6kl/htc_mbox.c:368:9: warning: incorrect type in assignment (different base types)
drivers/net/wireless/ath/ath6kl/htc_mbox.c:368:9:    expected restricted __le16 x
drivers/net/wireless/ath/ath6kl/htc_mbox.c:368:9:    got unsigned short [usertype]

Use put_unaligned_le16() so that the value is converted to little endian before
storing it to the header.

Compile tested only.

Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240320182449.3757215-2-kvalo@kernel.org