mmc: tmio: use EXPORT_SYMBOL_GPL

Use EXPORT_SYMBOL_GPL rather than the non _GPL variant as there seems to be
no reason not to.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>