mtd: pfow: remove unused variable

Fixes warning:

In file included from drivers/mtd/lpddr/qinfo_probe.c:31:0:
include/linux/mtd/pfow.h: In function ‘send_pfow_command’:
include/linux/mtd/pfow.h:104:6: warning: variable ‘chipnum’ set but not used [-Wunused-but-set-variable]
  int chipnum;
      ^

Signed-off-by: Brian Norris <computersforpeace@gmail.com>