Commit 51f2ded ("param: make return
value of param getter function const") made the parameter getter const,
but neglected to change the dev_add_param() function prototype in the
!CONFIG_PARAMETER case.

Fixes the following warning with CONFIG_NAND and !CONFIG_PARAMETER:

drivers/mtd/nand/nand_base.c: In function 'add_mtd_nand_device':
drivers/mtd/nand/nand_base.c:1720:2: warning: passing argument 4 of 'dev_add_param' from incompatible pointer type [enabled by default]
include/param.h:59:19: note: expected 'char * (*)(struct device_d *, struct param_d *)' but argument is of type 'const char * (*)(struct device_d *, struct param_d *)'

Signed-off-by: Josh Cartwright <joshc@eso.teric.us>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>