net/octeontx2: fix buffer size assignment

The elt_size field in mempool holds the size of one packet buffer.
It can be used to set the lpm_sizem1 field in rq context.

The lpb_sizem1 field in rq context is 12 bit, direct assignment
to it was causing overflow of value. Because of this errors
were observed while trying inline inbound with large packets.
This patch resolves the errors.

Fixes: 094fc8a3a1e2 ("net/octeontx2: add Rx queue setup and release")
Cc: stable@dpdk.org
Signed-off-by: Ankur Dwivedi <adwivedi@marvell.com>
Reviewed-by: Nithin Dabilpuram <ndabilpuram@marvell.com>
Reviewed-by: Jerin Jacob <jerinj@marvell.com>