rtw88: fix fw_fifo_addr check

The clang build reports this warning

fw.c:1485:21: warning: address of array 'rtwdev->chip->fw_fifo_addr'
  will always evaluate to 'true'
        if (!rtwdev->chip->fw_fifo_addr) {

fw_fifo_addr is an array in rtw_chip_info so it is always
nonzero.  A better check is if the first element of the array is
nonzero.  In the cases where fw_fifo_addr is initialized by rtw88b
and rtw88c, the first array element is 0x780.

Fixes: 0fbc2f0f ("rtw88: add dump firmware fifo support")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Tzu-En Huang <tehuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201011155438.15892-1-trix@redhat.com