ppc-aix osabi sniffer: Turn test of bfd flavour into assertion

ppc-aix osabi sniffer: Turn test of bfd flavour into assertion

Due to the way this function is registers, we know that given bfd's
flavour should always be bfd_target_xcoff_flavour, thus making
the former test always true, which means that this function should
always return GDB_OSABI_AIX, and never return GDB_OSABI_UNKNOWN.

This patch also fixes a typo detected by Tom Tromey that caused
the test itself to be completely ineffective.

gdb/ChangeLog (by Tom Tromey and Joel Brobecker):

        * rs6000-aix-tdep.c (rs6000_aix_osabi_sniffer): Replace
        inneffective if condition by gdb assertion.  Add function
        description comment.