mtd: mtdchar: handle chips that have user otp but no factory otp

Before this patch mtd_read_fact_prot_reg was used to check availability
for both MTD_OTP_FACTORY and MTD_OTP_USER access. This made accessing
user otp for chips that don't have a factory otp area impossible. So use
the right wrapper depending on the intended area to be accessed.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>