sys: fix implicit conversion uint32 to uint8

When running clang-tidy and logging module is used through any of the
LOG_ macro, an issue was raised in Z_CBPRINTF_PACK_ARG2 macro.

A uint32_t variable was copied into a _rws_buffer array with an implicit
cast (error: implicit conversion loses integer precision: 'uint32_t'
(aka 'unsigned int') to 'uint8_t' (aka 'unsigned char')).

Here we are adding an explicit cast to get rid of the implicit
conversion error.

Signed-off-by: Nathan Olff <nathan@kickmaker.net>