net: ethernet: Check that header is valid

Saw this crash with heavily loaded system in mimxrt1050_evk:

<err> os: ***** MPU FAULT *****
<err> os:   Data Access Violation
<err> os:   MMFAR Address: 0xc
<err> os: r0/a1:  0x80000ab0  r1/a2:  0x800f6a60  r2/a3:  0x00000000
<err> os: r3/a4:  0x800f72a0 r12/ip:  0x00000000 r14/lr:  0x6000eb43
<err> os:  xpsr:  0x41000000
<err> os: Faulting instruction address (r15/pc): 0x6000dc82
<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
<err> os: Current thread: 0x80001a18 (rx_workq)
<err> os: Halting system

Where the fault at 0x6000dc82 points to ethernet_recv()

	uint16_t type = ntohs(hdr->type);
6000dc82:	89ab      	ldrh	r3, [r5, #12]

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>