block: Simplify usb_msd_initfn() test for "can read bdrv key"

The old test assumes that "hotplugged" implies "we have a current
monitor for reading the key".  This is in fact true, but it's not
obviously true.

Aside: if it were false, we could pass a null pointer to
monitor_read_bdrv_key_start(), which would then crash.

The previous commit permits us to check for "we have a current
monitor" directly, so do that.
