net: stmmac: Unexport stmmac_rx_offset() from stmmac.h

stmmac_rx_offset() is referenced in stmmac_main.c only,
let's move it to stmmac_main.c.

Drop the inline keyword by the way, it is better to let the compiler
to decide.

Compile tested only.
No functional change intended.

Signed-off-by: Furong Xu <0x1207@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250107075448.4039925-1-0x1207@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>