samples: net: lwm2m: adjust BT RX buffer count /sizes for BT

Due to higher traffic demands when using 6lowpan, we should raise
the default BT RX buffer counts and sizes.

This avoids the following error on some HW:
[net/buf] [WRN] net_buf_alloc_len_debug: bt_buf_get_rx():4985:
Pool 0x20009424 low on buffers.
[net/buf] [WRN] net_buf_alloc_len_debug: bt_buf_get_rx():4985:
Pool 0x20009424 blocked for 0 secs

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>