bcm47xx: revert bgmac back to the old limited max frame size

bcm47xx: revert bgmac back to the old limited max frame size

Bumping max frame size has significantly affected network performance
and memory usage. It was done by upstream commit that first appeared in
the 5.7 release.

Allocating 512 (BGMAC_RX_RING_SLOTS) buffers, 10 k each, is clearly a
bad idea on 32 MiB devices. This commit fixes support for Linksys E1000
V2.1 which gives up after allocating ~346 such buffers running 5.15
kernel.

Ref: 230c9da963aa ("bcm53xx: revert bgmac back to the old limited max frame size")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>