caif-hsi: Set default MTU to 4096

Default MTU for CAIF HSI was wrongly set to 15 * 4092 bytes.
The patch sets default MTU size to 4096.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>