cmd: sf/nand: Print and return failure when 0 length is passed

For sf commands, when '0' length is passed for erase, update, write or
read, there might be undesired results. Ideally '0' length means nothing to
do.

So print 'ERROR: Invalid size 0' and return cmd failure when length '0' is
passed to sf commands. Same thing applies for nand commands also.

Example:

ZynqMP> sf erase 0 0
ERROR: Invalid size 0
ZynqMP> sf write 10000 0 0
ERROR: Invalid size 0
ZynqMP> sf read 10000 0 0
ERROR: Invalid size 0
ZynqMP> sf update 1000 10000 0
ERROR: Invalid size 0
ZynqMP>

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>