USBDevice: endpoint_add/read_start functions return value fix

[maciejbocianski]

Description

Fix value returned from USBDevice::endpoint_add/USBDevice::read_start functions

Pull request type

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

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1837
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6704/

Triggering tests

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

[mbed-ci]

Exporter Build : FAILURE

Build number : 1482
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6704/

[cmonr]

Eclipse:
JVM terminated. Exit code=1

ಠ_ಠ

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1485
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6704/

[mbed-ci]

Test : SUCCESS

Build number : 1644
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6704/1644