Staging:rtl8821ae: Drop memory allocation cast for kmalloc in hal_btc.c

Remove unnecessary cast for kmalloc, after running alloc_cast.cocci semantic patch.

Signed-off-by: Iulia Manda <iulia.manda21@gmail.com>
Reviewed-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>