samples/rpmsg: Replace print_hex_dump() with print_hex_dump_debug()

Replace the raw print_hex_dump() call in the rpmsg_sample_cb() function
with the equivalent print_hex_dump_debug() better suited for dynamic
debug. This switch allows flexibility of controlling this trace through
dynamic debug when enabled.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>