net: lib: zperf: fix kernel panic due to invalid thread prio

Compiling an application with CONFIG_NET_ZPERF=y leaving
CONFIG_ZPERF_WORK_Q_THREAD_PRIORITY at its default value would
systematically cause a kernel panic during thread initialization.

The Kconfig variable is NUM_PREEMPT_PRIORITIES by default. Application
threads may not define a priority lower than NUM_PREEMPT_PRIORITIES - 1,
though.

This change limits zperf's thread priority to a valid range. It does not
change the default value as it makes sense to default the thread
priority to the lowest possible value (which is NUM_PREEMPT_PRIORITIES)
but Kconfig does not allow for arithmentic. So the combination of
CLAMP() plus the Kconfig default will ensure min priority plus limit the
range to valid values no matter what has been defined as priority in
Kconfig.

Fixes: #59141

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>