rsi: fix integer overflow warning

gcc produces a harmless warning about a recently introduced
signed integer overflow:

drivers/net/wireless/rsi/rsi_91x_hal.c: In function 'rsi_prepare_mgmt_desc':
include/uapi/linux/swab.h:13:15: error: integer overflow in expression [-Werror=overflow]
  (((__u16)(x) & (__u16)0x00ffU) << 8) |   \
   ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
include/uapi/linux/swab.h:104:2: note: in expansion of macro '___constant_swab16'
  ___constant_swab16(x) :   \
  ^~~~~~~~~~~~~~~~~~
include/uapi/linux/byteorder/big_endian.h:34:43: note: in expansion of macro '__swab16'
 #define __cpu_to_le16(x) ((__force __le16)__swab16((x)))
                                           ^~~~~~~~
include/linux/byteorder/generic.h:89:21: note: in expansion of macro '__cpu_to_le16'
 #define cpu_to_le16 __cpu_to_le16
                     ^~~~~~~~~~~~~
drivers/net/wireless/rsi/rsi_91x_hal.c:136:3: note: in expansion of macro 'cpu_to_le16'
   cpu_to_le16((tx_params->vap_id << RSI_DESC_VAP_ID_OFST) &
   ^~~~~~~~~~~

The problem is that the 'mask' value is a signed integer that gets
turned into a negative number when truncated to 16 bits. Making it
an unsigned constant avoids this.

Fixes: eac4eed3 ("rsi: tx and rx path enhancements for p2p mode")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>