net: stmmac: move frag_size handling out of spin_lock

net: stmmac: move frag_size handling out of spin_lock

The upcoming patch will extract verification logic into a new module,
MMSV (MAC Merge Software Verification). MMSV will handle most FPE fields,
except frag_size. It introduces its own lock (mmsv->lock), replacing
fpe_cfg->lock.

Since frag_size handling remains in the driver, the existing rtnl_lock()
is sufficient. Move frag_size handling out of spin_lock_irq_save() to keep
the upcoming patch a pure refactoring without behavior changes.

Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Furong Xu <0x1207@gmail.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>