Fixed OBOE in async serial tx for NRF52 target, fixes #4002
Fixed OBOE in async serial tx for NRF52 target, fixes #4002

j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os