staging: rtl8188eu: fix incorrect ERROR tags from logs

Several lifecycle events in the rtl8188eu driver are logged using the
DBG_88E_LEVEL macro from rtw_debug.h, which is tagged as ERROR
regardless of the actual level.  Below are dmesg excerpts after loading
and unloading the module, the messages are misleading as there was no
error.

    [517434.916239] usbcore: registered new interface driver r8188eu
    [517435.680653] R8188EU: ERROR indicate disassoc
    [517437.122606] R8188EU: ERROR assoc success
    [517797.735611] usbcore: deregistering interface driver r8188eu
    [517797.736069] R8188EU: ERROR indicate disassoc

Remove the ERROR prefix from the logs.  After the patch, logs are:

    [517949.873976] usbcore: registered new interface driver r8188eu
    [517950.592845] R8188EU: indicate disassoc
    [517951.993973] R8188EU: assoc success
    [521778.784448] usbcore: deregistering interface driver r8188eu
    [521778.784838] R8188EU: indicate disassoc

Signed-off-by: Galo Navarro <anglorvaroa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>