net: shell: iface: Add link speed settings command

Add this to set link speed through net_shell.
Current zephyr version doesn't have user interface to change
speed, hence adding this to accomodate it. Full duplex will
be set by default for each link speed.

Able to change link speed using command below:
net iface set_link <iface idx> <speed> <optional:h(half)/f(full)>
eg: net iface set_link 1 100 h
    net iface set_link 1 10 f
    net iface set_link 1 1000

Able to set multiple link speed like below:
net iface set_link <iface idx> <speed1> <speed2> <optional:h/f>
eg: net iface set_link 1 10 100 h
    net iface set_link 1 1000 f 10

Signed-off-by: Ling Pei Lee <pei.lee.ling@intel.com>