Two mask registers are used in order to select which events have to
actually generate an interrupt on each IRQ line.

It seems that in the single-IRQ case it's assumed that the IRQs lines
are simply OR-ed, while the two mask registers are still present. The
driver still programs the two mask registers separately.

However the STM32 variant has only one IRQ, and also has only one mask
register.

This patch prepares for STM32 variant support by making the driver using
only one mask register.

This patch also optimize the MMCIMASK1 mask usage by caching it into
host->mask1_reg which avoid to read it into mmci_irq().

Tested only on STM32 variant. RFT for variants other than STM32
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>