mtd: maps: physmap: Store gpio_values correctly

When the gpio-addr-flash.c driver was merged with physmap-core.c the
code to store the current gpio_values was lost. This meant that once a
gpio was asserted it was never de-asserted. Fix this by storing the
current offset in gpio_values like the old driver used to.

Fixes: commit ba32ce95 ("mtd: maps: Merge gpio-addr-flash.c into physmap-core.c")
Cc: <stable@vger.kernel.org>
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Richard Weinberger <richard@nod.at>