staging: pi433: remove unnecessary calling rf69_set_mode()

Remove unnecessary rf69_set_mode() function call when rx is waiting for
a telegram. There is waste to call rf69_set_mode() twice for becoming
standby mode.

Signed-off-by: Sidong Yang <realwakka@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>