ath6kl: fix compilation warning in ath6kl_htc_pipe_conn_service

Fix the warning

drivers/net/wireless/ath/ath6kl/htc_pipe.c: In function
'ath6kl_htc_pipe_conn_service':
drivers/net/wireless/ath/ath6kl/htc_pipe.c:1293:26: warning: integer overflow
in expression [-Woverflow]

by giving a hint to compiler about unsigned nature of
HTC_CONN_FLGS_SET_RECV_ALLOC_MASK

Signed-off-by: Vladimir Murzin <murzin.v@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>