libffi: fix a typo (mips)

libffi: fix a typo (mips)

While code elsewhere checks for
MIPS_INSTRUCTION_SET == mips16e in order to decide how
to compile, hence the typo doesn't affect behaviour, the
intention was to set it to 'mips', as is done everywhere
else. Fixing the typo also helps to avoid confusion.

Signed-off-by: André Draszik <git@andred.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>