drivers: net: lan865x: Fix initialization of semaphores

This change fixes semaphores' definition. To be more specific - the
limit was wrongly set to UINT_MAX. With those changes - the 'tx_rx_sem'
now assures that only one execution path (i.e. receiving or sending
data) is executed at a time.

Moreover, the change in 'int_sem' now assures that this semaphore limit,
when reached, is saturated.

Signed-off-by: Lukasz Majewski <lukma@denx.de>