Tiva: Fix errors in low-level timer support

arch/arm/src/tiva/common/tiva_timerlib.c:
* tiva_gptm_configure() and tiva_gptm_release(): Fix error: Was calling
  tiva_emac_periphrdy() instead of tiva_gptm_periphrdy().

arch/arm/src/tiva/tiva_timer.h:
* Fix error in "abcde" description of bit flag defines.

arch/arm/src/tiva/common/tiva_timerlib.c

@@ -1747,7 +1747,7 @@ TIMER_HANDLE tiva_gptm_configure(const struct tiva_gptmconfig_s *config)
 
   /* Wait for the reset to complete */
 
-  while (!tiva_emac_periphrdy());
+  while (!tiva_gptm_periphrdy());
   up_udelay(250);
 
   /* Select the alternate timer clock source is so requested.  The general
@@ -1906,7 +1906,7 @@ void tiva_gptm_release(TIMER_HANDLE handle)
 
   /* Wait for the reset to complete */
 
-  while (!tiva_emac_periphrdy());
+  while (!tiva_gptm_periphrdy());
   up_udelay(250);
 
   /* Disable power and clocking to the GPTM module */

arch/arm/src/tiva/tiva_timer.h

@@ -98,8 +98,8 @@
 /* Flags bit definitions in configuration structures.  NOTE: not all flags
  * apply in all timer modes.  Applicable modes noted with:
  *
- *   a. 32-bit one shot timer
- *   b. 32-bit periodic timer
+ *   a. 16-bit one shot timer
+ *   b. 16-bit periodic timer
  *   c. 32-bit one shot timer
  *   d. 32-bit periodic timer
  *   e. 32-bit RTC timer