Staging: rtl8188eu: core: Use __func__ instead of function name

Replace the function name from format string with the
constant __func__ to avoid multiple changes in case
the name of the function will be modified.

Issue reported by checkpatch.pl script.

Signed-off-by: Georgiana Chelu <georgiana.chelu93@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>