staging: rtl8188eu: remove header file if_ether.h

"if_ether.h" is included in three files but not being used, so
remove "include/if_ether.h" header file and inclusion of this header file.

Signed-off-by: navin patidar <navin.patidar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>