Fix bug in Nordic SDK 14.2 SPI driver

[marcuschangarm]

Interrupts were not enabled before transmitting data leading to
missed interrupts.

Description

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1856
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6720/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : FAILURE

Build number : 1501
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6720/

[mbed-ci]

Test : FAILURE

Build number : 1664
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6720/1664

[marcuschangarm]

/morph export-build
/morph test

[marcuschangarm]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 1666
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6720/1666

[mbed-ci]

Exporter Build : ABORTED

Build number : 1503
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6720/

[0xc0170]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1504
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6720/