firmware_loader: Fix labels with comma for builtin firmware

firmware_loader: Fix labels with comma for builtin firmware

Some firmware images contain a comma, such as:
EXTRA_FIRMWARE "brcm/brcmfmac4334-sdio.samsung,gt-s7710.txt"
as Broadcom firmware simply tags the device tree compatible
string at the end of the firmware parameter file. And the
compatible string contains a comma.

This doesn't play well with gas:

drivers/base/firmware_loader/builtin/brcm/brcmfmac4334-sdio.samsung,gt-s7710.txt.gen.S: Assembler messages:
drivers/base/firmware_loader/builtin/brcm/brcmfmac4334-sdio.samsung,gt-s7710.txt.gen.S:4: Error: bad instruction `_fw_brcm_brcmfmac4334_sdio_samsung,gt_s7710_txt_bin:'
drivers/base/firmware_loader/builtin/brcm/brcmfmac4334-sdio.samsung,gt-s7710.txt.gen.S:9: Error: bad instruction `_fw_brcm_brcmfmac4334_sdio_samsung,gt_s7710_txt_name:'
drivers/base/firmware_loader/builtin/brcm/brcmfmac4334-sdio.samsung,gt-s7710.txt.gen.S:15: Error: can't resolve `.rodata' {.rodata section} - `_fw_brcm_brcmfmac4334_sdio_samsung' {*UND* section}
make[6]: *** [../scripts/Makefile.build:357: drivers/base/firmware_loader/builtin/brcm/brcmfmac4334-sdio.samsung,gt-s7710.txt.gen.o] Error 1

We need to get rid of the comma from the labels used by the
assembly stub generator.

Replacing a comma using GNU Make subst requires a helper
variable.

Cc: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20191115225911.3260-1-linus.walleij@linaro.org
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>