staging: r8188eu: Fix smatch warning in hal/usb_ops_linux.c

Smatch reports the following warning:
drivers/staging/rtl8188eu/hal/usb_ops_linux.c:614 usb_read_port() warn: variable dereferenced before check 'precvbuf' (see line 611)

Fixing this particular warning required some refactoring of the code.

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