redpine: Move card interrupt handling to RX thread

The interrupt handling of the RS911x is particularly heavy. For each RX
packet, the card does three SDIO transactions, one to read interrupt
status register, one to RX buffer length, one to read the RX packet(s).
This translates to ~330 uS per one cycle of interrupt handler. In case
there is more incoming traffic, this will be more.

The drivers/mmc/core/sdio_irq.c has the following comment, quote "Just
like traditional hard IRQ handlers, we expect SDIO IRQ handlers to be
quick and to the point, so that the holding of the host lock does not
cover too much work that doesn't require that lock to be held."

The RS911x interrupt handler does not fit that. This patch therefore
changes it such that the entire IRQ handler is moved to the RX thread
instead, and the interrupt handler only wakes the RX thread.

This is OK, because the interrupt handler only does things which can
also be done in the RX thread, that is, it checks for firmware loading
error(s), it checks buffer status, it checks whether a packet arrived
and if so, reads out the packet and passes it to network stack.

Moreover, this change permits removal of a code which allocated an
skbuff only to get 4-byte-aligned buffer, read up to 8kiB of data
into the skbuff, queue this skbuff into local private queue, then in
RX thread, this buffer is dequeued, the data in the skbuff as passed
to the RSI driver core, and the skbuff is deallocated. All this is
replaced by directly calling the RSI driver core with local buffer.

Signed-off-by: Marek Vasut <marex@denx.de>
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