drivers: ethernet: lan865x: Don't wait on semaphore if no memory for pkt

With the current approach, the driver prevents from TX transmission
when waiting on timeout (standard 100ms) for available memory to be
able to allocate memory for RX packet.

It is safe to just protect the part of reading chunks. In that way
pending TX transmission can be performed.

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