wilc1000: Convert module-global "isinit" to device-specific variable

Move static "isinit" variable to wilc_spi structure to allow multiple
WILC1000 devices per machine.

Signed-off-by: David Mosberger-Tang <davidm@egauge.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210828151346.3012697-1-davidm@egauge.net