mtd: pmcmsp: use kstrndup instead of kmalloc+strncpy

commit 906b268477bc03daaa04f739844c120fe4dbc991 upstream.

kernelci.org reports a warning for this driver, as it copies a local
variable into a 'const char *' string:

    drivers/mtd/maps/pmcmsp-flash.c:149:30: warning: passing argument 1 of 'strncpy' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]

Using kstrndup() simplifies the code and avoids the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>