gps: reduce delay by reducing the sleep time
gps: reduce delay by reducing the sleep time

This uses now the same sleep time logic as mavlink, depending on the
baudrate.

CPU usage on a Pixracer for different sleep times:

#num reads/sec    sleep time         CPU usage
17-18             2.8ms              0.233-0.31% (this PR)
12                5ms                0.155-0.3%
9-10              10ms               0.155-0.233%
6                 20ms               0.155-0.233% (previous)

PX4/Firmware,Aerotenna/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,Aerotenna/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,acfloria/Firmware,Aerotenna/Firmware,PX4/Firmware,krbeverx/Firmware,dagar/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,dagar/Firmware,Aerotenna/Firmware,acfloria/Firmware,krbeverx/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,Aerotenna/Firmware,acfloria/Firmware,krbeverx/Firmware,dagar/Firmware,PX4/Firmware,acfloria/Firmware,Aerotenna/Firmware,dagar/Firmware,dagar/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,krbeverx/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,krbeverx/Firmware