FMC: support carriers with no mezzanine

At registration and unregistration time, I was checking fmc->flags
for FMC_DEVICE_NO_MEZZANINE, to skip initialization and cleanup for
empty slots.  The check was wrong ("==" instead of "&") but
registration failed anyways (as expected) because we had no EEPROM.

This commit fixes one such checks and removes the other, so to
actually accept slots with no mezzanines. That's because the carrier
may offer some support anyways (the SPEC does), and working on the
carrier with no mezzanine-specific driver is common during
development.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Juan David Gonzalez Cobas <dcobas@cern.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>