If the Sefer Torah is open and the Kohen is reading the "Shema'", he is not permitted to interrupt his reading of the "Shema'" in order to go up to the Torah. Even if he is the only Kohen in the Synagogue he may not be called and another member of the congregation goes up in his place.
Even if the Torah has not yet been opened, they do not wait for him. This is because waiting for him would place an unnecessary burden on the congregation. Even if he was actually called up by name (perhaps they did not realize that he was reading the Shema') he does not interrupt his reading of the Shema' to go up to the Torah.
The same applies if he is in the middle of the blessings of the Shema'. However, if he is in the middle of the Pesuqei DeZimra he is permitted to go up if he is the only Kohen in the Synagogue.
(See Sh. 'A. 135: 5. Kaf HaHayim ibid, Oth 27, and 61: 24, 51 25-26)
