Bluetooth: shell: Allow passing more than one byte to gatt write

This makes it possible to pass different byte arrays to gatt write
making it usable to test attributes that take more than a single byte.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>