wifi: wilc1000: Fold chip_allow_sleep()/chip_wakeup() into wlan.c

Neither chip_allow_sleep()/chip_wakeup() is used outside of wlan.c .
Make both functions static and remove both the exported symbol and
entries from wlan.h .

Make chip_allow_sleep() return error code in preparation for the
follow up patches.

Move acquire_bus() and release_bus() to avoid forward declaration
of chip_allow_sleep()/chip_wakeup().

Reviewed-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20241004114551.40236-3-marex@denx.de