chacha20-ppc: fix implicit conversion between vector types warnings

* cipher/chacha20-ppc.c (chacha20_ppc_blocks4)
(chacha20_ppc_blocks4, chacha20_poly1305_ppc_blocks4): Explicitly
convert return values from vec_cmplt.

Commit solves new warning type:

warning: Implicit conversion between vector types (''vector bool
unsigned int' (vector of 4 'unsigned int' values)' and
''vector4x_u32' (vector of 4 'unsigned int' values)') is deprecated.
In the future, the behavior implied by '-fno-lax-vector-conversions'
will be the default. [-Wdeprecate-lax-vec-conv-all]

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>