uhd: Avoid reallocation of buffers every read

Buffer size is based on num of chans and rxBuffer size is based on num
of chans and rx_spp, and both are available and set during open(), so no
need to waste time allocating and freeing them everytime we read from
the device.

Change-Id: I8c881c9c303c80f323825d85a924d74b76d2ce47
2 files changed