net: w5100: enable to support sleepable register access interface

SPI transfer routines are callable only from contexts that can sleep.

This adds ability to tell the core driver that the interface mode
cannot access w5100 register on atomic contexts.  In this case,
workqueue and threaded irq are required.

This also corrects timeout period waiting for command register to be
automatically cleared because the latency of the register access with
SPI transfer can be interfered by other contexts.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Mike Sinkovsky <msink@permonline.ru>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>