tc-i386.c gcc10 warning fix

tc-i386.c gcc10 warning fix

.../gas/config/tc-i386.c: In function ‘md_parse_option’:
.../gas/config/tc-i386.c:11441:9: error: implicit conversion from ‘enum <anonymous>’ to ‘enum <anonymous>’ [-Werror=enum-conversion]
11441 |  vexwig = evexw0;
      |         ^
.../gas/config/tc-i386.c:11443:9: error: implicit conversion from ‘enum <anonymous>’ to ‘enum <anonymous>’ [-Werror=enum-conversion]
11443 |  vexwig = evexw1;
      |         ^

	* config/tc-i386.c (md_parse_option): Fix warning on vexwig assignment.

(cherry picked from commit 40c9c8deb94be6576f5729172dce117cbe155856)