Bluetooth: RFCOMM: Move BT_RFCOMM_BUF_SIZE to rfcomm.h

The buffer `BT_RFCOMM_BUF_SIZE` is used to define the TX buffer size of
TX pool.

In current implementation, the TX buffer size of RFCOMM cannot be
calculated due to the macro `BT_RFCOMM_BUF_SIZE` is defined in internal
header file `rfcomm_internal.h`.

Move the macro `BT_RFCOMM_BUF_SIZE` form internal header file
`rfcomm_internal.h` to interface `rfcomm.h`.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>