mmc: cavium-octeon: Fix interrupt enable code

mmc: cavium-octeon: Fix interrupt enable code

OCTEON SoCs with CIU3 do not have interrupt masking local to the MMC
bus interface.  Unfortunately, some even have a diagnostic register at
the same address of the enable register, which causes the interrupts
to fire immediately if stored to, thus breaking the driver.  The proper
action on these SoCs is not to touch this register.

Fixes: 01d95843335c ("mmc: cavium: Add MMC support for Octeon SOCs.")
Signed-off-by: David Daney <david.daney@cavium.com>
[jglauber@cavium.com: removed point after subject line]
Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>