Bug 18934: Preventing warns in Admin -> SMS providers

Bug 18934: Preventing warns in Admin -> SMS providers

This patch initialises the $op variable to prevent the warns

To test:
1) Go to admin -> sms providers
2) Notice warns in error log
3) Apply patch and refresh page
4) Notice warns are gone

Sponsored-by: Catalyst IT
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 88ab0ef5791d2b428399690c505ac42aba4616df)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit adbd961d82fa6aebcc3f93f2dec6af1c8127aab9)
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>