bhyve: Fix sign compare warnings in the e1000 device model.

Adding a bare constant to a uint16_t promotes to a signed int which
triggers these warnings. Changing the constant to be explicitly
unsigned instead promotes the expression to unsigned int.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37485

(cherry picked from commit e7cd5ffff88f1f4dfba2693041cc78fcf3613fba)