flash/nand/core: fix clang static analyzer warning

flash/nand/core: fix clang static analyzer warning

core.c:446:
The left operand of '>>' is a garbage value

There are many places where an error code returned from nand->controller
operations are ignored. To keep the change minimal, the error checks are
added only to reading of extended nand info as it was suspected
to be the cause of the warning.

Addition of the error checks did not fix the warning.
scan-build-9 report was inspected and IMHO the warning is bogus:
the term (nand->device->erase_size == 0) cannot give false at line 395
and then evaluate true at line 462. Fixed by zeroing id_buff.

Change-Id: I97ed7ce0fdf1aa23d746d5fb898bacd050e20ae8
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5518
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>