firmware: arm_sdei: Remove sdei_is_err()

sdei_is_err() is only called by sdei_to_linux_errno(). The logic of
checking on the error number is common to them. They can be combined
finely.

This removes sdei_is_err() and its logic is combined to the function
sdei_to_linux_errno(). Also, the assignment of @err to zero is also
dropped in invoke_sdei_fn() because it's always overridden afterwards.
This shouldn't cause functional changes.

Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: James Morse <james.morse@arm.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20200922130423.10173-2-gshan@redhat.com


Signed-off-by: Will Deacon <will@kernel.org>