Bluetooth: Fix false-positive "uninitialized" compiler warning

Some gcc versions don't seem to be able to properly track the flow of
the smp_cmd_pairing_random() function and end up causing the following
types of (false-positive) warnings:

 smp.c:1995:6: warning: ‘nb’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   err = smp_g2(smp->tfm_cmac, pkax, pkbx, na, nb, &passkey);
 smp.c:1995:6: warning: ‘na’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   err = smp_g2(smp->tfm_cmac, pkax, pkbx, na, nb, &passkey);
       ^
 smp.c:1995:6: warning: ‘pkbx’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   err = smp_g2(smp->tfm_cmac, pkax, pkbx, na, nb, &passkey);
       ^
 smp.c:1995:6: warning: ‘pkax’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   err = smp_g2(smp->tfm_cmac, pkax, pkbx, na, nb, &passkey);

This patch fixes the issue by moving the pkax/pkbx and na/nb
initialization earlier in the function.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>