Bluetooth: Shell: BR: Set user data size for rfcomm tx pool

The user data size of the RFCOMM tx pool is zero. There is not enough
space to put the tx_meta data. Use CONFIG_BT_CONN_TX_USER_DATA_SIZE to
set the data size for the RFCOMM tx pool.

Signed-off-by: Make Shi <make.shi@nxp.com>