nvmecontrol: Fix condition when print number of Firmware Slots and Firmware Slot1 Readonly.

The Number of Firmware Slots should never be zero. So, a Firmware Slot 1
should always exist. For that reason, always print the Number of
Firmware Slots and the Firmware Slot 1 Read-Only value.

Reviewed by: imp
Approved by: manu (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34700