staging: rtl8188eu: Remove unneeded cast

Fix Coccinelle alert:

drivers/staging//rtl8188eu/os_dep/usb_intf.c:336:13-27: WARNING: casting value returned by memory allocation function to (struct adapter *) is useless.

This issue was detected by using the Coccinelle software.

Signed-off-by: Christopher Diaz Riveros <chrisadr@gentoo.org>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>