drivers: lora: shell: fix invalid range in config bw param

'bw' parameter in 'lora config' command has a range of 0 to INT8_MAX.
However possible values of 'bw' (125, 250, 500) don't fit this range
and we need to extend it to 0 to INT16_MAX.

This fix has been tested on custom board with STM32L4 and SX1276.

Signed-off-by: Petr Sharshavin <sharshavin@mail.ru>