gstdio: simplify _g_win32_get_mode_alias

Do an in-place update on the wide-character mode string
10 jobs for win32-gstdio-minor in 24 minutes and 10 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #333490
cross-android_api21_arm64

00:02:52

passed #333491
cross-android_api28_arm64

00:02:17

passed #333492
cross-mingw64

00:04:08

passed #333488
debian-stable-x86_64

00:04:28

passed #333487
fedora-x86_64

00:07:02

passed #333489
G_DISABLE_ASSERT

00:04:27

passed #333493
win32
msys2-mingw32

00:08:54

passed #333494
win32
vs2017-x64

00:09:05

 
  Coverage
passed #333495
coverage

00:01:26

78.1%
 
  Analysis
passed #333496
scan-build

00:04:38