TARGET_STM: remove warning and fix typo on SPI

[ABOSTM]

Description

Remove TARGET_STM compilation warning for SPI.
By the way fix typo in comment.

Pull request type

[x ] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 6 of 11 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_cloud-client-test
• jenkins-ci/mbed-os-ci_exporter
• jenkins-ci/mbed-os-ci_mbed2-build-ARM
• jenkins-ci/mbed-os-ci_greentea-test
• jenkins-ci/mbed-os-ci_mbed2-build-IAR
• jenkins-ci/mbed-os-ci_mbed2-build-GCC_ARM

[0xc0170]

CI restarted

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 2
Build artifacts