staging: isdn: remove assignment in if conditionals

staging: isdn: remove assignment in if conditionals

Remove variable assignment in if statements in
drivers/staging/isdn/avm/b1.c.
Issues reported by checkpatch.pl as:
ERROR: do not use assignment in if condition

Also refactor code around some if statements to remove comparisons
to NULL and unnecessary braces in single statement blocks.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Link: https://lore.kernel.org/r/20191011072044.7022-1-wambui.karugax@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>