drivers: ethernet: sam0: Increase RX buffer count by one

The SAM GMAC driver is not able to use all of the receive buffers
concurrently. Frames larger than (buffer size) * (buffer count - 1) are
silently dropped. Fix it by adding one to MAIN_QUEUE_RX_DESC_COUNT.

Fixes #55701

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