drivers: ethernet: adin2111: fix generic spi frame reception

Fixes a bug that results in double RX buffer read from ADIN
when generic SPI protocol is used.

Actual frame size to be read must be RX_FSIZE - HEADER.
If we read less amount of bytes, then ADIN raises an IRQ or
signals via Pn_RX_RDY that there is another frame available, as
we fail to read the whole frame.

The fix for "Still some length to go 2" is still present, we drop
the CRC32 bytes from the frame prior allocation and network
stack pass.

Signed-off-by: Georgij Cernysiov <geo.cgv@gmail.com>