memory: omap-gpmc: return meaningful error codes in gpmc_cs_set_timings()

The callers of gpmc_cs_set_timings() expect to receive -ERRNO on errors
and they pass further what they have received.

However gpmc_cs_set_timings() was returning -1 (equal to -EPERM) which
does not make sense in this context.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200724074038.5597-21-krzk@kernel.org