platform/x86: oxpec: Add a lower bounds check in oxp_psy_ext_set_prop()

platform/x86: oxpec: Add a lower bounds check in oxp_psy_ext_set_prop()

The "val->intval" variable is an integer which comes from the user.  This
code has an upper bounds check but the lower bounds check was
accidentally omitted.  The write_to_ec() take a u8 value as a parameter
so negative values would be truncated to positive values in the 0-255
range.

Return -EINVAL if the user passes a negative value.

Fixes: 202593d1e86b ("platform/x86: oxpec: Add charge threshold and behaviour to OneXPlayer")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Antheas Kapenekakis <lkml@antheas.dev>
Link: https://lore.kernel.org/r/aBSE71VKfBlQg_fZ@stanley.mountain
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>