staging: rtl8712: Remove redundant cast

struct firmware::data has type const u8*, as does *ppmappedfw, so the
cast to u8* is unnecessary and slightly confusing.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>