wifi: shell: Rectify data type of beacon interval and DTIM period

Data types of beacon interval and DTIM period are defined
incorrectly.

dtim_period=1-255, so, unsigned char 802.11-202: 9.4.2.5.1 General.

beacon_interval=2 bytes, so, unsigned short: 802.11-2020 - 9.4.1.3
Beacon Interval field.

Signed-off-by: Ravi Dondaputi <ravi.dondaputi@nordicsemi.no>