enic: Simplify RX handler function

Split up RX handler functions in preparation for moving
to a page pool based implementation.

No functional changes.

Co-developed-by: Nelson Escobar <neescoba@cisco.com>
Signed-off-by: Nelson Escobar <neescoba@cisco.com>
Co-developed-by: Satish Kharat <satishkh@cisco.com>
Signed-off-by: Satish Kharat <satishkh@cisco.com>
Signed-off-by: John Daley <johndale@cisco.com>
Link: https://patch.msgid.link/20250205235416.25410-3-johndale@cisco.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>