smsc(4): update to mention Microchip

Microchip Technology acquired SMSC in 2012, and all current products
and datasheets refer to the devices supported by this driver as
Microchip parts. Mention SMSC in a parenthetical comment to explain
the driver's name.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45115

(cherry picked from commit 7ef6ce51742d44a7375ccbaeda4cc64e034c4816)