ehea: fix skb_frag_size typo

Commit 9e903e08 ("net: add skb frag size accessors") introduced a
typo in ehea driver.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>