samples: lora: receive: update SNR unit to dB

Signal-to-noise ratio is a unitless quantity, so its unit is dB, not
dBm (dBm is the power relative to 1 milliwatt). At the same time output
the received payload using `LOG_HEXDUMP_INF`, instead of limiting the
sample solely to string payloads.

Fixes #82414

Signed-off-by: Jordan Yates <jordan@embeint.com>