hw: mcu: pic32mx470f512h: Reduce number of UART TX interrupts
hw: mcu: pic32mx470f512h: Reduce number of UART TX interrupts

See commit e3bf3c2ff4542494ce8530caf0a8fde99b7a0c92 for
explanations.

Signed-off-by: Francois Berder <59eaf4bb0211c66c3d7532da6d77ecf42a779d82@outlook.fr>

mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,IMGJulian/incubator-mynewt-core