staging: r8188eu: Fix smatch warnings in core/rtw_recv.c

Smatch reports the following:
drivers/staging/rtl8188eu/core/rtw_recv.c:211 rtw_free_recvframe() warn: variable dereferenced before check 'precvframe' (see line 207)
drivers/staging/rtl8188eu/core/rtw_recv.c:1803 amsdu_to_msdu() warn: variable dereferenced before check 'sub_skb' (see line 1784)

The code was rearranged to eliminate the dereference before the check.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>