net: ena: Update LLQ header length in ena documentation

LLQ entry length is 128 bytes. Therefore the maximum header in
the entry is calculated by:
tx_max_header_size =
LLQ_ENTRY_SIZE - DESCRIPTORS_NUM_BEFORE_HEADER * 16 =
128 - 2 * 16 = 96

This patch updates the documentation so that it states the correct
max header length.

Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>