mtd: spi-nor: wait for SR_WIP to clear on initial unlock

Fixup a piece leftover by commit 32321e95 ("mtd: spi-nor: wait until
lock/unlock operations are ready"). That commit made us wait for the WIP
bit to settle after lock/unlock operations, but it missed the open-coded
"unlock" that happens at probe() time.

We should probably have this code utilize the unlock() routines in the
future, to avoid duplication, but unfortunately, flash which need to be
unlocked don't all have a proper ->flash_unlock() callback.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Stas Sergeev <stsp@users.sourceforge.net>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Tested-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>