Bluetooth: GOEP: Improve the MTU configuration

The MTU of the GOEP should be not less than 255. So, if the transport
is RFCOMM, the CONFIG_BT_BUF_ACL_RX_SIZE should be not less than 264.
It includes,
- 255 bytes for the minimum MTU of GOEP,
- 4 bytes for L2CAP Header,
- 5 bytes for RFCOMM header and FCS.

And if the transport is L2CAP, the CONFIG_BT_BUF_ACL_RX_SIZE should
be not less than 259.
It includes,
- 255 bytes for the minimum MTU of GOEP,
- 4 bytes for L2CAP Header.

Add Kconfig `BT_GOEP_RFCOMM_MTU` to configure the maximum size for
RFCOMM transport. The range of `BT_GOEP_RFCOMM_MTU` is
`[264, BT_RFCOMM_L2CAP_MTU]`. And the GOEP MTU via RFCOMM transport
should be in the range `[255, (BT_GOEP_RFCOMM_MTU-9)]`.

Add Kconfig `BT_GOEP_L2CAP_MTU` to configure the maximum size for
L2CAP transport. The range of `BT_GOEP_L2CAP_MTU` is
`[259, BT_BUF_ACL_RX_SIZE]`. And the GOEP MTU via L2CAP transport
should be in the range `[255, (BT_GOEP_L2CAP_MTU-4)]`.

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