mmc: tmio: remove indirection of 'execute_tuning' callback

After all the previous refactorization, we can now populate mmc_ops
directly and don't need a layer inbetween. The NULL-pointer check and
the error printout are already done by the MMC core.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20200820132538.24758-7-wsa+renesas@sang-engineering.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>