USB2NET : SR9800 : change SR9800_BULKIN_SIZE from global to static

Smatch reports this issue
sr9800.h:166:53: warning: symbol 'SR9800_BULKIN_SIZE' was not declared. Should it be static?

Global variables should not be defined in header files.
This only works because sr9800.h in only included by sr9800.c
Change the storage-class specifier to static.
And since it does not change add type qualifier const.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>