mmc: tmio: introduce mask for 'always 1' bits

Some variants (namely Renesas SDHI) have bits in the STATS and IRQ_MASK
registers which are 'always 1' and should be written as such. Introduce
a seperate mask for this and apply it whenever such a register is
written.

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