fastboot: Fix fastboot reboot fail by changing functions order

It was revealed that when the fastboot_tx_write_str function is called
without the previously initialized fastboot_func->in_req->complete field,
a copy of in_req will be sent to the I/O requests queue without
an initialized field.

Moving a piece of code with the initializing of the
fastboot_func->in_req->complete field above transmit_tx allows to solve
this problem.

Fixes: 65c96757 "usb: fastboot: Convert USB f_fastboot to shared fastboot"
Signed-off-by: yurii.pidhornyi <yurii.pidhornyi@globallogic.com>