net: stmmac: make SPH enable/disable to be configurable

SPH functionality splits header and payload according to split mode and
offsef fields (SPLM and SPLOFST). It is beneficials for Linux network
stack RX processing however it adds a lot of complexity in XDP
processing.

So, this patch makes the split-header (SPH) capability of the controller
is stored in "priv->sph_cap" and the enabling/disabling of SPH is decided
by "priv->sph".

This is to prepare initial XDP enabling for stmmac to disable the use of
SPH whenever XDP is enabled.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>