kernel: fix bugs added with mac-address-ascii support

kernel: fix bugs added with mac-address-ascii support

1. Check for -EPROBE_DEFER
If it occurs we have to return immediately. Trying other properties
could result in another error and ignoring -EPROBE_DEFER which has a
special meaning.

2. Check for read result
Assuming property->read() success can result in NULL pointer
dereference. It happens e.g. for "mac-address" with NVMEM cell
containing invalid MAC.

3. Simplify code
Don't move cell reading & nvmem_cell_put() into a loop. Simplify loop
code.

Fixes: ecd81de7a5ab ("ath79: add nvmem cell mac-address-ascii support")
Cc: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>