bhyve/vga.c: fix atc_color_select_67 bit shift

Gcc noticed that the result of the bit shift is always zero. Shift so
that the ATC_CS_C67 bits end up in bits 6 & 7.

Reviewed by: grehan, tychon
Approved by: markj (mentor)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11775