enic: Move RX functions to their own file

Move RX handler code into its own file in preparation for further
changes. Some formatting changes were necessary in order to satisfy
checkpatch but there were 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/20250205235416.25410-2-johndale@cisco.com


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