sd: sdhci: mask transfer mode register value

In SDHCI protocol, the transfer mode register is defined
to be of 6 bits. Mask its value with '0x0037' so that an
invalid value could not be assigned.

Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20170214185225.7994-2-ppandit@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
