AP_Proximity: avoid use of int16_t-read call

[peterbarker]

Replaces #27059 and is careful not to change any logic.

[tridge]

@peterbarker looks good.
btw, after the last discussion I realised that uart->available() is actually constrained to the size of the readbuf on all HALs. That means <= 4k unless someone has done an unusual begin()