powerpc/pseries: Use ARRAY_SIZE to iterate over firmware_features_table array

When iterating over the entries in firmware_features_table we only need
to go over the actual number of entries in the array instead of declaring
it to be bigger and checking to make sure there is a valid entry in every
slot.

This patch removes the FIRMWARE_MAX_FEATURES #define and replaces the
array looping with the use of ARRAY_SIZE().

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>