Fix typos in comments and documentation (#750)

* Fix typos in comments and documentation

drivers/usbdev/composite.c

@@ -720,7 +720,7 @@ static int composite_setup(FAR struct usbdevclass_driver_s *driver,
        * Non-Standard Class Requests
        **********************************************************************/
 
-       /* Class implementations should handle there own interface and endpoint
+       /* Class implementations should handle their own interface and endpoint
         * requests.
         */
 

drivers/wireless/ieee80211/bcm43xxx/bcmf_sdio.c

@@ -259,8 +259,8 @@ int bcmf_probe(FAR struct bcmf_sdio_dev_s *sbus)
       goto exit_error;
     }
 
-  /* Default device clock speed is up to 25 Mhz
-   * We could set EHS bit to operate at a clock rate up to 50 Mhz.
+  /* Default device clock speed is up to 25 MHz
+   * We could set EHS bit to operate at a clock rate up to 50 MHz.
    */
 
   SDIO_CLOCK(sbus->sdio_dev, CLOCK_SD_TRANSFER_4BIT);

drivers/wireless/spirit/lib/spirit_radio.c

@@ -90,7 +90,7 @@ static const uint8_t g_vectc_bandval[4] =
   SYNT0_BS_6, SYNT0_BS_12, SYNT0_BS_16, SYNT0_BS_32
 };
 
-/* It represents the available channel bandwidth times 10 for 26 Mhz xtal.
+/* It represents the available channel bandwidth times 10 for 26 MHz xtal.
  * NOTE: The channel bandwidth for others xtal frequencies can be computed
  * since this table multiplying the current table by a factor
  * xtal_frequency/26e6.