firmware: tegra: Reduce stack usage

Building the bpmp-debugfs driver for Arm results in a warning for stack usage:

drivers/firmware/tegra/bpmp-debugfs.c:321:16: error: stack frame size of 1224 bytes in function 'bpmp_debug_store' [-Werror,-Wframe-larger-than=]
static ssize_t bpmp_debug_store(struct file *file, const char __user *buf,

It should be possible to rearrange the code to not require two separate
buffers for the file name, but the easiest workaround is to use dynamic
allocation.

Fixes: 5e37b9c1 ("firmware: tegra: Add support for in-band debug")
Link: https://lore.kernel.org/all/20201204193714.3134651-1-arnd@kernel.org/


Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[treding@nvidia.com: consistently return NULL on failure]
Signed-off-by: Thierry Reding <treding@nvidia.com>