eth/ksdk: Use k_sem_init() initial count

Remove the existing code that iterates over k_sem_give() and setup the
initial counter directly, take the opportunity to set the maximum count.

Change-Id: Ib91ea263567ff761e4953c142a22a56658efe293
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>