[Fix] Fix the error of access type setting

[Lance52259]

What this PR does / why we need it:
Access type and access level hasn't default value, program will throw a Error when SFS doing upodation.
In huaweicloud SFS API, access level has a default value 'rw' and access type is 'cert'. When program doing updation,
Terraform will doing read and use it to compare with state config. Then, program find a difference because of access type
set null in updation and state config is not empty. The API doesn't accept null values, so, the Error happiend.

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

NONE

Special notes for your reviewer:

Release note:

1. set access_level and access_type to computed.
2. use d.GetOk() instead of d.Get() and empty check because of d.GetOk() contains d.Get() and empty check.

PR Checklist

• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccSFSFileSystemV2_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run=TestAccSFSFileSystemV2_basic -timeout 360m -parallel 4
=== RUN   TestAccSFSFileSystemV2_basic
=== PAUSE TestAccSFSFileSystemV2_basic
=== CONT  TestAccSFSFileSystemV2_basic
--- PASS: TestAccSFSFileSystemV2_basic (103.77s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       103.818s