On Galois groups of linearized polynomials related to the general linear group of prime degree

Let $L(x)$ be any $q$-linearized polynomial with coefficients in ${\mathbb{F}}_q$, of degree $q^n$. We consider the Galois group of $L(x)+tx$ over ${\mathbb{F}}_q(t)$, where $t$ is transcendental over ${\mathbb{F}}_q$. We prove that when $n$ is a prime, the Galois group is always $GL(n,q)$, except when $L(x)=x^{q^n}$. Equivalently, we prove that the arithmetic monodromy group of $L(x)\mathrm{/}x$ is $GL(n,q)$.