Staging: rtl8192u: Remove function prototype from .c file

Remove function prototype from r8192U_core.c since it is
declared in the r8192U.h header and this is included in r8192U_core.c

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>