drivers: ethernet: sam0: Fix receive buffer initialization

The SAM GMAC driver is not utilizing the entire receive buffer, causing
large packets to be dropped. Fix it by setting Receiver Packet Buffer
Memory Size Select to Full (RXBMS = 3) in the GMAC_DCFGR register.

Fixes #55701

Signed-off-by: Brandon Del Bel <delbel@umn.edu>