bluetooth: hci: spi: warn about l2cap MTU values

`CONFIG_BT_L2CAP_TX_MTU` drives the maximum supported MTU on a Bluetooth
connection, but the SPI backend also imposes its own hidden limits.

Display a warning if a value is chosen that can result in failures to
send. This is done here instead of as a range on `BT_L2CAP_TX_MTU` as
ranges on that symbol are already defined in terms of software features,
which would conflict with this check.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>