staging: rtl8188eu: use __func__ instead of function name

Replace occurrence of the function name in a string by reference to
__func__, to improve robustness and to conform to the Linux kernel coding
style. Issue found using checkpatch.

Signed-off-by: Santha Meena Ramamoorthy <santhameena13@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>