firmware: fix directory creation rule matching with make 3.82

Attempting to run 'firmware_install' with CONFIG_USB_SERIAL_TI=y when
using make 3.82 results in an error

make[2]: *** No rule to make target `/lib/firmware/./', needed by
`/lib/firmware/ti_3410.fw'.  Stop.

It turns out make 3.82 is picky when matching directory names with
trailing slashes as a result, where make 3.81 would handle this
correctly make 3.82 does not find the rule needed to create the
directory.

The './' seen in the error is added by $(dir) for firmware which
resides in the base firmware src directory, such as
ti_3410.fw.ihex. By performing $(dir) after we prepend the
$(INSTALL_FW_PATH) we can ensure we don't end up with a './' in the
middle of the path and the directory will be properly created.

This change works with make 3.81 and should work with previous
versions as well.

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>