smp: shell: consume all pending buffers in smp_shell_process()

smp_shell_process() is called only once after receiving new bytes over
shell. If multiple MCUMGR frames were received over UART one after the
other, then calling smp_shell_process() resulted in consuming only the
first one. All subsequent frames were not processed unless there was
some more RX traffic.

Process received frames in smp_shell_process() in a loop until there is
no frame left. This will make sure that received packets are not stalled
waiting for more RX traffic to trigger processing again.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>