ds1307: use the correct Microchip part number in enum and device description

During a minor refactoring two years ago (part of 2486b446), the newly
created enum used the wrong part number - MCP7491x instead of MCP7941x. The
device description string got the same transposition of digits.

This change swaps the digits back to what they should be.

Reviewed by: emaste, tsoome, imp
Differential Revision: https://reviews.freebsd.org/D44436