mraa_board_t: fix operator in *_mmap_unsetup

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/intel_edison_fab_c.c

@@ -507,7 +507,7 @@ mraa_intel_edsion_mmap_unsetup()
         return MRAA_ERROR_INVALID_RESOURCE;
     }
     munmap(mmap_reg, mmap_size);
-    mmap_reg == NULL;
+    mmap_reg = NULL;
     close(mmap_fd);
     return MRAA_SUCCESS;
 }

src/intel_galileo_rev_d.c

@@ -44,7 +44,7 @@ mraa_intel_galileo_g1_mmap_unsetup()
         return MRAA_ERROR_INVALID_RESOURCE;
     }
     munmap(mmap_reg, mmap_size);
-    mmap_reg == NULL;
+    mmap_reg = NULL;
     close(mmap_fd);
     return MRAA_SUCCESS;
 }

src/intel_galileo_rev_g.c

@@ -197,7 +197,7 @@ mraa_intel_galileo_g2_mmap_unsetup()
         return MRAA_ERROR_INVALID_RESOURCE;
     }
     munmap(mmap_reg, mmap_size);
-    mmap_reg == NULL;
+    mmap_reg = NULL;
     close(mmap_fd);
     return MRAA_SUCCESS;
 }