[R][BACKPORT-2.2] backport lint fix

## What changes were proposed in this pull request?

backport part of the commit that addresses lintr issue

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes apache#21325 from felixcheung/rlintfix22.

R/pkg/R/sparkR.R

@@ -196,7 +196,7 @@ sparkR.sparkContext <- function(
 
     # Don't use readString() so that we can provide a useful
     # error message if the R and Java versions are mismatched.
-    authSecretLen = readInt(f)
+    authSecretLen <- readInt(f)
     if (length(authSecretLen) == 0 || authSecretLen == 0) {
       stop("Unexpected EOF in JVM connection data. Mismatched versions?")
     }