ath6kl: Fix ath6kl_bmi_read_hi32 macro

tmp may be used uninitialized if ath6kl_bmi_read() returns an error.

Signed-off-by: Frederic Danis <frederic.danis@linux.intel.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>