Fix 'make clean' and .gitignore for firmware/ directory.

Provide a .gitignore file, and fix a typo which prevented some of the
generated binary files from being removed on 'make clean'.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Reported-and-tested-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

firmware/.gitignore

@@ -0,0 +1,6 @@
+*.gen.S
+*.fw
+*.bin
+*.csp
+*.dsp
+ihex2fw

firmware/Makefile

@@ -56,7 +56,7 @@ fw-shipped-$(CONFIG_USB_SERIAL_KEYSPAN_USA28X) += keyspan/usa28x.fw
 fw-shipped-$(CONFIG_USB_SERIAL_KEYSPAN_USA49W) += keyspan/usa49w.fw
 fw-shipped-$(CONFIG_USB_SERIAL_KEYSPAN_USA49WLC) += keyspan/usa49wlc.fw
 else
-fw-shipped- := keyspan/mpr.fw keyspan/usa18x.fw keyspan/usa19.fw	\
+fw-shipped- += keyspan/mpr.fw keyspan/usa18x.fw keyspan/usa19.fw	\
 	keyspan/usa19qi.fw keyspan/usa19qw.fw keyspan/usa19w.fw		\
 	keyspan/usa28.fw keyspan/usa28xa.fw keyspan/usa28xb.fw		\
 	keyspan/usa28x.fw keyspan/usa49w.fw keyspan/usa49wlc.fw