serial: 8250_EXAR: fix duplicate Kconfig text and add missing help text

In commit d0aeaa83 ("serial: exar:
split out the exar code from 8250_pci") the exar driver got its own
Kconfig.  However the text for the new option was never changed from
the original 8250_PCI text, and hence it appears confusing when you
get asked the same question twice:

  8250/16550 PCI device support (SERIAL_8250_PCI) [Y/n/m/?] (NEW)
    8250/16550 PCI device support (SERIAL_8250_EXAR) [Y/n/m] (NEW)

Adding to the confusion, is that there is no help text for this new
option to indicate it is specific to a certain family of cards.

Fix both issues at the same time, as well as the space vs. tab issues
introduced in the same commit.

Fixes: d0aeaa83 ("serial: exar: split out the exar code from 8250_pci")
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-serial@vger.kernel.org
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: NSudip Mukherjee <sudipm.mukherjee@gmail.com>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/Kconfig

@@ -128,9 +128,13 @@ config SERIAL_8250_PCI
 	  by the parport_serial driver, enabled with CONFIG_PARPORT_SERIAL.
 
 config SERIAL_8250_EXAR
-        tristate "8250/16550 PCI device support"
+	tristate "8250/16550 Exar/Commtech PCI/PCIe device support"
 	depends on SERIAL_8250_PCI
 	default SERIAL_8250
+	help
+	  This builds support for XR17C1xx, XR17V3xx and some Commtech
+	  422x PCIe serial cards that are not covered by the more generic
+	  SERIAL_8250_PCI option.
 
 config SERIAL_8250_HP300
 	tristate