hw/ssi/aspeed_smc: Rename 'max_slaves' variable as 'max_peripherals'

In order to use inclusive terminology, rename max_slaves
as max_peripherals.

Patch generated using:

  $ sed -i s/slave/peripheral/ \
        hw/ssi/aspeed_smc.c include/hw/ssi/aspeed_smc.h

One line in aspeed_smc_read() has been manually tweaked
to pass checkpatch.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201012124955.3409127-2-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>