nandtest: fix --reads argument

nandtest: fix --reads argument

The --reads option specifies the argument as optional, but doesn't check
for a null optarg, which means that nandtest segfaults when run as
"nandtest --reads".

Fix this by making the argument required, and changing the help text to
not specify it as optional. Argument -r already specifies the argument
as required, so we fix this inconsistency too.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Acked-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>