lib8390: Replace panic() call with BUILD_BUG_ON

Replace panic() call in lib8390.c with BUILD_BUG_ON()
since checking the size of struct e8390_pkt_hdr should
happen at compile-time.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>