drivers/fmc: hide fmc operations behind helpers

This gave us more freedom to change/add/remove operations without
recompiling all device driver.

Typically, Carrier board implement the fmc operations, so they will not
use these helpers.

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
Tested-by: Pat Riehecky <riehecky@fnal.gov>
Acked-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>