rtlwifi: fix incorrect use of usb_alloc_coherent with usb_control_msg

Incorrect use of usb_alloc_coherent memory as input buffer to usb_control_msg
can cause problems in arch DMA code, for example kernel BUG at
'arch/arm/include/asm/dma-mapping.h:321' on ARM (linux-3.4).

Change _usb_writeN_sync use kmalloc'd buffer instead.

Cc: stable@vger.kernel.org
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>