firmware: Use $(quote) in the Makefile

The literal " confuses syntax highlighting in vim.

Cc: David Woodhouse <dwmw2@infradead.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

firmware/Makefile

@@ -4,10 +4,10 @@
 
 # Create $(fwabs) from $(CONFIG_EXTRA_FIRMWARE_DIR) -- if it doesn't have a
 # leading /, it's relative to $(srctree).
-fwdir := $(subst ",,$(CONFIG_EXTRA_FIRMWARE_DIR))
+fwdir := $(subst $(quote),,$(CONFIG_EXTRA_FIRMWARE_DIR))
 fwabs := $(addprefix $(srctree)/,$(filter-out /%,$(fwdir)))$(filter /%,$(fwdir))
 
-fw-external-y := $(subst ",,$(CONFIG_EXTRA_FIRMWARE))
+fw-external-y := $(subst $(quote),,$(CONFIG_EXTRA_FIRMWARE))
 
 # There are three cases to care about:
 # 1. Building kernel with CONFIG_FIRMWARE_IN_KERNEL=y -- $(fw-shipped-y) should