wifi: rtw89: correct base HT rate mask for firmware

Coverity reported that u8 rx_mask << 24 will become signed 32 bits, which
casting to unsigned 64 bits will do sign extension. For example,
putting 0x80000000 (signed 32 bits) to a u64 variable will become
0xFFFFFFFF_80000000.

The real case we meet is:
  rx_mask[0...3] = ff ff 00 00
  ra_mask = 0xffffffff_ff0ff000

After this fix:
  rx_mask[0...3] = ff ff 00 00
  ra_mask = 0x00000000_ff0ff000

Fortunately driver does bitwise-AND with incorrect ra_mask and supported
rates (1ss and 2ss rate only) afterward, so the final rate mask of
original code is still correct.

Addresses-Coverity-ID: 1504762 ("Unintended sign extension")

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240809072012.84152-5-pkshih@realtek.com