staging: r8188eu: Fix smatch warning in os_dep/osdep_service.c

Smatch reports the following warning:
drivers/staging/rtl8188eu/os_dep/osdep_service.c:634 rtw_change_ifname() warn: variable dereferenced before check 'padapter' (see line 630)

The dereference in question is deferred until the validity of 'padapter' is verified.

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