IIOD: Write the channel mask in one write_all() call
IIOD: Write the channel mask in one write_all() call

Previously it was impossible to know the number of USB transfers
required to fetch the wole channel mask, when IIOD was used over USB.

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

sensarliar/libiio,gburca/libiio,gburca/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,sensarliar/libiio,analogdevicesinc/libiio,sensarliar/libiio,gburca/libiio