rt2x00: Fix word size of rt2500usb MAC_CSR19 register.

The register is 16 bits wide, not 32.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Cc: <stable@vger.kernel.org>
Acked-by: Ivo Van Doorn <ivdoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>