The slot control register is modified as follows:

    (1) Read the register value
    (2) Change the value
    (3) Write the value to the register

Those must be done atomically, otherwise writing to control register
would cause an unexpected result.
Signed-off-by: NKenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: NKristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>