mailbox: mailbox-test: avoid reading iomem twice

Don't pass mmio region as source to print_hex_dump() and then
again to memcpy_fromio(). Do it once and give print_hex_dump()
the buffer we just read the data in.

Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>